Show friendlier error message for zero donation amount

Add a non-zero check in the non-empty category for 'amount', so
the user sees 'Please enter your donation amount' instead of
'Please correct the errors in your donation amount'.  Prevent
more complex validation error messages from clobbering simpler ones.

Bug: 56657
Change-Id: I4738fbf3257e632260771317ec7eb18924280c2d
Merge "Show friendlier error message for zero donation amount"