Make metadata error message more informative

[hammannr]

This is just a tiny improvement to make the error message of a metadata mismatch in the NeymanConstructor more useful for checking which files to look into.

[coveralls]

Pull Request Test Coverage Report for Build 11316411924

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 89.632%

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Files with Coverage Reduction	New Missed Lines	%
alea/parameters.py	1	90.91%

Totals
Change from base Build 11316175646:	-0.05%
Covered Lines:	1703
Relevant Lines:	1900

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💛 - Coveralls