Check iterator error on Drop where applicable

[hypnoglow]

Fixes #432

[coveralls]

Pull Request Test Coverage Report for Build 885

• 8 of 24 (33.33%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 55.639%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
database/cockroachdb/cockroachdb.go	1	3	33.33%
database/firebird/firebird.go	1	3	33.33%
database/mysql/mysql.go	1	3	33.33%
database/postgres/postgres.go	1	3	33.33%
database/ql/ql.go	1	3	33.33%
database/redshift/redshift.go	1	3	33.33%
database/sqlcipher/sqlcipher.go	1	3	33.33%
database/sqlite3/sqlite3.go	1	3	33.33%

Totals
Change from base Build 883:	-0.1%
Covered Lines:	3044
Relevant Lines:	5471

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💛 - Coveralls

[dhui]

Thanks for the PR!