stack trace when description or summary contain non-ascii characters

I think disallowing non-ASCII characters is by design due to the message I get:

yaml.reader.ReaderError: unacceptable character #x1f4a9: special characters are not allowed
  in "snapcraft.yaml", position 145

This is clear and concise but it comes at the end of a traceback:

Traceback (most recent call last):
  File "/snap/snapcraft/924/bin/snapcraft", line 11, in <module>
    load_entry_point('snapcraft==2.37', 'console_scripts', 'snapcraft')()
  File "/snap/snapcraft/924/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 565, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/snap/snapcraft/924/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2631, in load_entry_point
    return ep.load()
  File "/snap/snapcraft/924/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2291, in load
    return self.resolve()
  File "/snap/snapcraft/924/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2297, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/cli/__main__.py", line 19, in <module>
    run(prog_name='snapcraft')
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/core.py", line 1043, in invoke
    return Command.invoke(self, ctx)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/cli/__init__.py", line 124, in run
    ctx.forward(lifecyclecli.commands['snap'])
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/core.py", line 553, in forward
    return self.invoke(cmd, **kwargs)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/cli/lifecycle.py", line 140, in snap
    project_options, directory=directory, output=output)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/internal/lifecycle/_packer.py", line 45, in snap
    execute('prime', project_options)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/internal/lifecycle/_runner.py", line 61, in execute
    config = snapcraft.internal.load_config(project_options)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/internal/project_loader/__init__.py", line 24, in load_config
    return Config(project_options)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/internal/project_loader/_config.py", line 98, in __init__
    snapcraft_yaml = _snapcraft_yaml_load(self.snapcraft_yaml_path)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/snapcraft/internal/project_loader/_config.py", line 267, in _snapcraft_yaml_load
    return yaml.load(fp)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/yaml/__init__.py", line 70, in load
    loader = Loader(stream)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/yaml/loader.py", line 34, in __init__
    Reader.__init__(self, stream)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/yaml/reader.py", line 85, in __init__
    self.determine_encoding()
  File "/snap/snapcraft/924/lib/python3.6/site-packages/yaml/reader.py", line 135, in determine_encoding
    self.update(1)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/yaml/reader.py", line 169, in update
    self.check_printable(data)
  File "/snap/snapcraft/924/lib/python3.6/site-packages/yaml/reader.py", line 144, in check_printable
    'unicode', "special characters are not allowed")
yaml.reader.ReaderError: unacceptable character #x1f4a9: special characters are not allowed
  in "snapcraft.yaml", position 145

This message should be friendlier.

I used snapcraft 2.37 from the beta channel snap (revision 924).

Here's my snapcraft.yaml:

name: hello-roadmr-1
version: 2017-12-11-02
summary: say hello more nicely
confinement: strict
grade: stable
description: hello-roadmr's friendly test 💩

architectures:
  - amd64

apps:
  hello:
    command: echo "hello"

parts:
  empty:
    plugin: nil

Note I've appended a 💩 character to the end of the description.

Interestingly, though, a few emoji *are* accepted; for instance, the snowflake (❄️).