Improve alignment exception error reporting

[pmachapman]

As a part of sillsdev/serval#893, this PR improves error reporting to give details on the ArgumentException that caused the CorpusAlignmentException, and fixes a NullReferenceException when one of the rows referenced in the CorpusAlignmentException is null.

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This change is 

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.78%. Comparing base (d1390f7) to head (9b3afc7).

Files with missing lines	Patch %	Lines
src/SIL.Machine/Corpora/NParallelTextCorpus.cs	0.00%	6 Missing ⚠️
...rc/SIL.Machine/Corpora/CorpusAlignmentException.cs	0.00%	4 Missing ⚠️
src/SIL.Machine/Corpora/ParallelTextCorpus.cs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   72.79%   72.78%   -0.02%     
==========================================
  Files         424      424              
  Lines       36309    36314       +5     
  Branches     5006     5006              
==========================================
  Hits        26431    26431              
- Misses       8770     8775       +5     
  Partials     1108     1108              

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[ddaspit]

@ddaspit reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on Enkidu93).

[Enkidu93]

@Enkidu93 reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on pmachapman).