Added helpful error message if an async function is passed to trio.to_thread_run_sync

[guilledk]

This functionality was previously added to the trio.from_thread functions in #1513

Closes #1573

[pquentin]

Thanks! Two minor things left.

[codecov]

Codecov Report

Merging #1575 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1575      +/-   ##
==========================================
- Coverage   99.69%   99.69%   -0.01%     
==========================================
  Files         111      109       -2     
  Lines       13866    13817      -49     
  Branches     1059     1056       -3     
==========================================
- Hits        13824    13775      -49     
  Misses         27       27              
  Partials       15       15              

Impacted Files	Coverage Δ
trio/_threads.py	100.00% <100.00%> (ø)
trio/tests/test_threads.py	100.00% <100.00%> (ø)
trio/tests/test_exports.py	97.22% <0.00%> (-0.15%)	⬇️
trio/__init__.py	100.00% <0.00%> (ø)
trio/tests/test_deprecate.py	100.00% <0.00%> (ø)
trio/_deprecated_subprocess_reexports.py
trio/_deprecated_ssl_reexports.py
trio/_core/_run.py	99.76% <0.00%> (+<0.01%)	⬆️