Say which variable a compiled expression was not given

[Rafael-SOWNet]

"a * x".ToEntity().Compile("x");
// KeyNotFoundException: The given key 'a' was not present in the dictionary

That names neither the variable, nor the compilation, nor anything a caller of Compile could act on. Both compilers did it — the one into a FastExpression and the one into a Linq expression tree — each by reaching into its own dictionary of arguments and letting the lookup fail.

Now

UncompilableNodeException: a is not among the variables the expression is being compiled over, which are x
UncompilableNodeException: y is not among the variables the expression is being compiled over, which are none

A compiled expression is a function of the variables it was compiled over, so one it does not mention is not something it can be given a value for. UncompilableNodeException is what both compilers already raise for a matrix or a derivative, so this is the exception a caller would already be catching.

Constants are unaffected: pi and e are substituted for their values before any variable is looked up, so "pi * x" still compiles over x alone.

Verification

13 tests, 8 of which fail without the change — 5 of those on the fast compiler, 3 on the Linq one. 4009 unit tests and 127 F# tests on both target frameworks, none failing.

No issue is filed for this; I found it sweeping the compilers against interpreted evaluation.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.42%. Comparing base (90c00a8) to head (ae64f4a).
⚠️ Report is 57 commits behind head on master.

Files with missing lines	Patch %	Lines
...Functions/Compilation/IntoLinq/IntoLinqCompiler.cs	90.90%	0 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #688      +/-   ##
==========================================
- Coverage   80.99%   80.42%   -0.58%     
==========================================
  Files         155      156       +1     
  Lines       13687    12917     -770     
  Branches     1957     2122     +165     
==========================================
- Hits        11086    10388     -698     
+ Misses       1990     1921      -69     
+ Partials      611      608       -3     

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