Fix logging of early import errors

[happz]

In some conditions, exceptions raised while importing plugins could have been reported as just the exception message, without any traceback. This was caused by the necessary loggers being defined in tmt.cli package, which must be imported correctly first for the loggers to be available. But, tmt.cli runs plugin discovery, and if for whatever reason import of a plugin fails, subsequent call to show_exception() would also raise an exception, because it would try to import unavailable tmt.cli.EXCEPTION_LOGGER, resulting in the most simple error logging implemented.

By moving bootstrap and exception loggers into their own module, both plugin discovery spawned from tmt.cli and the error reporting code can reach loggers they need, as their existence no longer depends on tmt.cli and plugins being imported successfully.

Make a small typo in tmt/config/themes/default.yaml, e.g. change restructuredtext-literal to restructuredtext-literal-. tmt lint will crash with a single line or output:

Failed to import the 'tmt.steps.discover.fmf' module from '/home/happz/git/tmt'.

With this patch, one gets full chain of errors:

$ tmt lint

Failed to import the 'tmt.steps.discover.fmf' module from '/home/happz/git/tmt'.

The exception was caused by 1 earlier exceptions

Cause number 1:

    Invalid theme configuration.

    The exception was caused by 1 earlier exceptions

    Cause number 1:

        1 validation error for Theme
        restructuredtext-admonition-warning-
          extra fields not permitted (type=value_error.extra)

Pull Request Checklist

• [x] implement the feature