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
Updating approximate currency conversion rates

Change-Id: I464ef774a9ab6dd903e1fd32c18adaed751d5edd
Don't depend on mediawiki; every RL module implicitly does

Change-Id: I9e6536a01219ba34c91dcea8d9d303b17f16c972
Localisation updates from https://translatewiki.net.

Change-Id: I53d65be436ec18b5430d2da188dd66848b31380c