Display error message when bad config is loaded

If a bad configuration file is submitted to rebar3 shell, display the
following error:

    ===> The configuration file submitted could not be read and will be
    ignored.

And keep going otherwise rather than silently failing.

While crash-fast is usually a good mechanism, the shell so far is very
tolerant of failures from apps to boot and whatnot, so this feels
appropriate.

Fixes #1019

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