QgsDistanceArea methods should raise QgsCsException when errors occur

[nyalldawson]

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.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit ba80cd4)

[troopa81]

Can we get rid of some (all of) those TODOs? here for instance, we have access to QgisApp visibleMessageBar()

[nyalldawson]

I'm happy for someone else to do that, but right now it's not my priority. There's no regression here at least, the behaviour will be identical to current behaviour

[uclaros]

Plugins might crash with an unhandled exception though. Should we somehow advertise this change to plugin developers? Mailing list? Other means?

[nyalldawson]

@uclaros

Plugins might crash with an unhandled exception though.

I realise that, but I think that's preferable to this API silently giving incorrect measurements. At least this way the plugin authors will be advised and make the decision themselves on the best way to handle this situation.

Should we somehow advertise this change to plugin developers? Mailing list? Other means?

This kind of thing usually goes into the PyQGIS part of the changelog

[qgis-bot]

@nyalldawson

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