The error messages for a theme that failed to be deserialized (or
otherwise failed to load) were covered up by the context/with_context
calls:
* The log message for a bad theme configured in config.toml would only
say "Failed to deserilaize theme"
* Selecting a bad theme via :theme would show "Theme does not exist"
With these changes, we let the TOML deserializer errors bubble up, so
the error messages can now say the line number of a duplicated
key - and that key's name - when a theme fails to load because of a
duplicated key.
Providing a theme which does not exist to :theme still gives a helpful
error message: "No such file or directory."
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