Make it easier to override errors/helptext templates in custom renderers

[akx]

By moving the template names to class-level variables in the renderer class, it's easier for subclasses to just change the rendered template without having to reimplement the if/else logic in e.g. FormRenderer.render_errors.

[coveralls]

Pull Request Test Coverage Report for Build 3512104018

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 97.415%

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Totals
Change from base Build 3512102161:	0.009%
Covered Lines:	593
Relevant Lines:	605

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💛 - Coveralls