[ActiveRecord] Fix DB connection issues during assets:precompile

[codealchemy]

Closes #3469

When initializing abstract (ActiveRecord) models, #table_exists? is used and calls through the connection, which (in the case of a precompile step) can raise errors other than the ActiveRecord::NoDatabaseError that's currently handled.

I believe this should cover all PostgreSQL and MySQL connection errors handled by Rails.

[coveralls]

Pull Request Test Coverage Report for Build 1843895617

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.889%

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Totals
Change from base Build 1834034935:	0.0%
Covered Lines:	3685
Relevant Lines:	3843

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💛 - Coveralls

[mshibuya]

Looks great, thanks 🙌

[ha4gu]

Now bin/rails assets:precompile works, thank you so much!