Quote values in error messages

[tylerjw]

In preparing slides for my talk on generate_parameter_library I produced this error message:

  what():  Invalid value set during initialization for parameter 'my_string': 
Parameter 'my_string' with the value place is not in the set {world, base, home}

I think this would be much clearer if all the values in the error are quoted. The value place is kind of just hidden there in the middle.

[codecov]

Codecov Report

Merging #96 (574076d) into main (0fbf42c) will not change coverage.
The diff coverage is 100.00%.

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##             main      #96   +/-   ##
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  Coverage   99.05%   99.05%           
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  Files          10       10           
  Lines         211      211           
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  Hits          209      209           
  Misses          2        2           

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rolling	99.05% <100.00%> (ø)

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Files	Coverage Δ
include/rsl/parameter_validators.hpp	98.93% <100.00%> (ø)

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[MarqRazz]

I didn't test but the changes look good.