Setting invalid value is not an exception

EnuperatedStringProperty threw an exception when given bad input rather than return an error message. This fixes that problem and adds tests to exercise the behavior.

The original issue is that bringing up the algorithm dialog for Rebin crashed the dialog so users couldn't run it. This fixes the crash, but the widget for the property is wrong and will be fixed soon.

There is no associated issue, but it is associated with EWM7925

To test:

Start mantidworkbench
Open the dialog for Rebin
See that it doesn't crash

This does not require release notes because it fixes an issue introduced during the development cycle.

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Functional Tests

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mantidproject / mantid