Add optional error handling without exception.

[pavel-jurasek]

This PR fix several things:

• Fix warning related to last version RSpec
• Fix duplication for spec_helper require statment
• Fix problem with using non bang method for raising exceptions (param raise exception)
• Add ability to handler error within request block

[mattt]

Thanks for your patch, @pavel-jurasek-hypofriend-de. Apologies for the delay in reviewing this.

I'm currently triaging the most pressing issues (primarily Sinatra 2.0 compatibility) for the next release, but am definitely interested in incorporating this into the next major version.

In the meantime, I'm happy to report that the rspec issue you identified is now fixed with 1d811d9.