Make modifiying/adding unique indexes and narrowing cols work.
Add export exception for column narrowing and fix tests.
Merge remote-tracking branch 'origin/master' into ENG-6071-emptytables2

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Fix assert() vs fail() bug and patch up test.
Merge remote-tracking branch 'origin/master' into ENG-6071-emptytables2

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ENG-6071: Take Izzy's feedback.
Merge pull request #1757 from VoltDB/ENG-6071-emptytables2

Eng-6071 Allow column narrowing and unique index changes when tables are empty