CLN: Remove unused Exceptions
There are at least 3 `Exception`s that are _never_ raised anywhere, so they can be removed without issue. (There are other exceptions that probably should be removed or refactored, but this is just a basic commit to make sure that these don't get used going forward.)

CLN: Remove 'DateConversionError' that is never raised anywhere

CLN: Remove TimeSeriesError that was never used

CLN: Remove unused 'PanelError'
