die if submit_form() called with invalid form_id

[oalders]

Fixes #286

[coveralls]

Pull Request Test Coverage Report for Build 291

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.5%) to 93.223%

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Files with Coverage Reduction	New Missed Lines	%
lib/WWW/Mechanize.pm	10	92.82%
<!--	Total:	10		-->

Totals
Change from base Build 287:	0.5%
Covered Lines:	729
Relevant Lines:	782

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💛 - Coveralls

[oalders]

Thanks @skaji!