CLN: Improve Exceptions in core/common, io/excel and core/format

Related to #3954.

Cleanups to Exceptions in core/common, io/excel, core/format. A few places I added `# pragma: no cover`, because it looked like the code was generally unreachable. A few cleanups in tests/test_common and better error message in `_mut_exclusive` (which needs to be used in Panel.reindex later on).

Specific places to look at (you can also grep for `# pragma: no cover` in the diff):

* core/common - lines 860 and 1162
* core/format - lines 359 and 823
* io/excel - lines 73, 382