Throw meaningful error when form.get is called with errors

Fixes #1658

When a form is bound with missing fields, it throws an unhelpful
IllegalStateException with the message "No value".  Now it will instead
report the errors in the exception.
Added test for throwing an exception from Form.get
Merge pull request #3539 from jroper/1658-form-get-error

Throw meaningful error when form.get is called with errors