Error message improvements

[thumphries]

This issue will be a checklist for every ambiguous and/or confusing error message BMX spits out.

• Helper / Function errors on too many / too few arguments are arcane
• Helper / Function Alternative instance produces misleading messages (just prints the last failed match) - would be nice to handle this, or at least improve nullable/optional
• Dynamic partial prints the wrong thing when custom context is not a context Type error (expected partial, actually list)
• Helpers have strict arity requirements, these should be explained in the error message and remaining args printed out

[charleso]

Left "Rendering error: Rendering error:\n  Error applying helper\n    Too many arguments (1 unused)\n\n    In helper 'numberSummary' invoked at line 1, col 10\n    In partial 'modules/number' invoked at line 175, col 20\n    In helper 'each' invoked at line 146, col 14\n    In helper 'if' invoked at line 116, col 6\n\n" /= Right ()