Corrected error messages in getValidatedActivityOptions

[jakobht]

What changed?

• Changed the error messages in getValidatedActivityOptions so they are correct
• Removed redundant line of code
• Added tests for the new error messages

Why? The error messages were slightly incorrect, the redundant line did not do anything

How did you test it? I made new unit tests which checks the error messages

Potential risks

• The redundant line might have some strange side effect I didn't catch, but I'm pretty sure it's just a leftover from a refactor as it is the first thing called in the function called on the line below
• Changing the error message changes the interface, so if some users are checking against the strings their code could break.

[CLAassistant]

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