Delete unused exception

[Michael1993]

Proposed commit message:

Remove PioneerException (#500)

In #403 we moved our utility class into an internal package and
removed a reflection-based solution that copied our utility. The
reflection-based solution used `PioneerException` on failure to 
signal the user that Pioneer failed to initialise itself. Because
that class was deleted and `PioneerException` is not used elsewhere,
we can safely remove it.

PR: #500

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PR checklist

• A prepared commit message exists

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