Add warning when schema is not supported

[lawrence-mbf]

Motivation

Fixes #499

How to test the behavior?

Attempt to call nwbRead on a file using a non-standard schema.

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[codecov]

Codecov Report

Merging #531 (c2cd517) into master (286d6b5) will decrease coverage by 0.06%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
- Coverage   87.71%   87.66%   -0.06%     
==========================================
  Files         132      132              
  Lines        5584     5592       +8     
==========================================
+ Hits         4898     4902       +4     
- Misses        686      690       +4     

Files Changed	Coverage Δ
nwbRead.m	77.14% <44.44%> (-2.24%)	⬇️

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