extensions: ignore exceptions from an extension's `getversion()` method
This method is usually called when there's a stacktrace being generated, or with
`hg version -v`. Raising another exception risks mangling the bug report info.
I hit this issue when trying to add the method to the keyring extension to
report the version of the extension and the underlying module, and ran into
demandimport issues prior to py3.8. It seems like a wise thing to do anyway,
though unfortunately there's no convenient `ui` object around to issue a
warning. Use 'unknown' to signal that it tried to report a version and failed,
unlike the default case of printing nothing.
Differential Revision: https://phab.mercurial-scm.org/D10540
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