fix errorhandling of LoggingHandler.emit

[beniwohli]

When a LogRecord has a non-string as the .msg attribute,
just trying to concatenate it with "\n" leads to an error.

A similar problem also existed in the LogBook handler.

This error was first reported in #1340, but it wasn't obvious due to a chained exception.

closes #1340

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• Start Time: 2022-06-08T08:48:34.660+0000

• Duration: 30 min 24 sec

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (68/68)	💚
Files	100.0% (227/227)	💚
Classes	100.0% (227/227)	💚
Lines	89.574% (17165/19163)	👍 0.007
Conditionals	76.632% (3122/4074)	👎 -0.002