Informative error message for sampler_diagnostics() with fixed_param

[jgabry]

closes #1032

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Ensure there's an informative error message when calling sampler_diagnostics() when fixed_param=TRUE.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Columbia University

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license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

Merging this without review since it’s super simple and I think other devs are pretty busy