Handle MWContentSerializationException in JsonDumpGenerator

This can happen if there is a corrupt entity in the database,
but should not cause the entire dump to fail.  The entity
can be skipped with an error message, as done for other error
situations.

Bug: 69846
Change-Id: I541fb68cd8697c8e9e6617c4cc8ca556a3e03238
Add method for generating json of individual entities in JsonDumpGenerator

split from the generateDump method

Change-Id: I0012acec9f3126ed07b069c3679a09ff503c82eb
protected -> private in JsonDumpGenerator

Change-Id: I385b8739c3dffb73d93f71710cf34cf113561516