Catch errors in completeness framework

[roed314]

With this PR, errors in the completeness framework print a warning message and log an error to the flasklog, instead of causing a server error. We can find and fix the underlying problems by examining the flasklog.

Compare:

• https://www.lmfdb.org/EllipticCurve/Q/?conductor_type=multiple&conductor=7

• http://localhost:37777/EllipticCurve/Q/?conductor_type=multiple&conductor=7

• https://www.lmfdb.org/ModularForm/GL2/Q/holomorphic/?level_type=multiple&level=23&weight=1

• http://localhost:37777/ModularForm/GL2/Q/holomorphic/?level_type=multiple&level=23&weight=1

[roed314]

@AndrewVSutherland we can use app.logger to print to the flasklog. I'm going to make another issue for cleaning up the loggers.

[roed314]

@AndrewVSutherland if you can review this and #6870 I can make a PR to alpha, dev and web.

[AndrewVSutherland]

This works, but I note that there are no timestamps in the flasklog. I don't think it's worth delaying this to fix/change that so I am merging.