Update numeric question validity checks

[deundrewilliams]

Resolves #2012

The initial issue was that the editor allows saving when a margin of error question is created with an answer of 0 and an error type of 'Percent', which would cause a blank screen error when viewing the assessment. This also occurs if any other input for numeric questions is invalid (range invalid, answer can't be parsed to a number, etc.). The first part of these changes adds a validity check to make sure that a margin of error question doesn't have an answer of 0 with an error type of 'Percent'. The other part adds a check before saving that checks the validity of all input and select fields. If any are invalid, the module won't be saved and an alert will be shown, so this will prevent any type of invalid input from being saved.

[FrenjaminBanklin]

This is a succinct change that works great. Well done!

I think I'd like to see this embellished a bit - rather than a standard browser alert, maybe using a fancier-looking component that provides specific details about any existing issues and potentially can move focus to the issues for quicker recovery.

For now, this is a great start.