[#208] Make `failUnexpected` produce the same message in Indigo and Lorentz

Description

Problem: failUnexpected produces two different error messages between its Indigo and Lorenz version:

[mt|Unexpected: |] <> msg

[mt|Internal: |] <> msg

Solution: Make Indigo and Lorentz versions of failUnexpected produce the same message.

Related issue(s)

Resolves #208 (closed)

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