Add MigrationRequiredException

[nirinchev]

Fixes #1448

[coveralls-official]

Pull Request Test Coverage Report for Build 7423689685

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 87.145%

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Totals
Change from base Build 7400751707:	-0.02%
Covered Lines:	3654
Relevant Lines:	4193

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💛 - Coveralls

[nielsenko]

Looks good.

One thing. Previously we mostly just threw RealmException on errors. We might benefit from using a similar pattern of creating a separate exception type in other situations as well.

[nirinchev]

Yes, creating dedicated exceptions is worthwhile, but I'd probably do it on demand rather than all at once.