Text.Pandoc.App: Throw errors rather than exiting.

These are caught (and lead to exit) in pandoc.hs, but
other uses of Text.Pandoc.App may want to recover in another
way.

Added PandocAppError to PandocError (API change).
This is a stopgap:  later we should have a separate constructor
for each type of error.

Also fixed uses of 'exit' in Shared.readDataFile, and
removed 'err' from Shared (API change).

Finally, removed the dependency on extensible-exceptions.

See #3548.

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