Instead of just silently return "0", which is misleading and
can result in data corruption/incorrect analysis results. Better
to be safe and force callers to handle transformation errors
appropriately.
In this case we:
- raise QgsProcessingExceptions when the failed measurement is coming
from a processing tool, so that the user is forced to deal with the
issue and we aren't providing meaningless/misleading measurements
- report evaluation errors if the measurement is coming from a
QGIS expression, so the user must appropriately handle the situation
- for all other cases we currently just write a console error and
maintain the current behavior of treating the measurement length
as 0. TODO notes have been added to handle this cases better.
