Display proper build errors

[johanhenriksson]

Getting an ugly stack trace when the base image dockerfile is invalid:

-- BASE -------------------------------------------------------------------------------------------------------------------------------------------
{'message': 'dockerfile parse error line 15: unknown instruction: WGET'}
Traceback (most recent call last):
  File "/Users/johanh/dev/dopeking2/.venv/bin/cowait", line 33, in <module>
    sys.exit(load_entry_point('cowait', 'console_scripts', 'cowait')())
  File "/Users/johanh/dev/pipeline/cowait/__main__.py", line 14, in main
    app(obj=config)
  File "/Users/johanh/dev/dopeking2/.venv/lib/python3.7/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/johanh/dev/dopeking2/.venv/lib/python3.7/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/johanh/dev/dopeking2/.venv/lib/python3.7/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/johanh/dev/dopeking2/.venv/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/johanh/dev/dopeking2/.venv/lib/python3.7/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/johanh/dev/dopeking2/.venv/lib/python3.7/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/johanh/dev/pipeline/cowait/cli/app/image.py", line 35, in build
    push=push,
  File "/Users/johanh/dev/pipeline/cowait/cli/commands/build.py", line 83, in build
    raise RuntimeError('Failed to build base image')
RuntimeError: Failed to build base image