Improve error messaging when invalid syntax is passed to the -m option

[jeffreyrack]

Closes #4051

Now when running pytest using the -m option without valid Python syntax, the error message will end with a clear statement as to why:

INTERNALERROR> SyntaxError: unexpected EOF while parsing (<string>, line 1)
INTERNALERROR> Marker expression must be valid Python!

[codecov]

Codecov Report

Merging #4057 into master will decrease coverage by 0.82%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4057      +/-   ##
==========================================
- Coverage   94.47%   93.65%   -0.83%     
==========================================
  Files         109      109              
  Lines       23803    23805       +2     
  Branches     2359     2356       -3     
==========================================
- Hits        22488    22294     -194     
- Misses       1004     1188     +184     
- Partials      311      323      +12

Flag	Coverage Δ
#doctesting	?
#linux	93.54% <100%> (-0.79%)	⬇️
#nobyte	?
#numpy	?
#pexpect	?
#py27	91.56% <100%> (-1.04%)	⬇️
#py34	92.01% <100%> (-0.09%)	⬇️
#py35	?
#py36	91.99% <100%> (-0.68%)	⬇️
#py37	?
#trial	?
#windows	90.86% <100%> (-2.96%)	⬇️
#xdist	?

Impacted Files	Coverage Δ
src/_pytest/mark/legacy.py	98% <100%> (+0.12%)	⬆️
testing/test_mark.py	97.07% <100%> (+0.02%)	⬆️
testing/python/approx.py	79.76% <0%> (-19.85%)	⬇️
testing/test_unittest.py	84.9% <0%> (-14.16%)	⬇️
src/_pytest/unittest.py	74.11% <0%> (-12.36%)	⬇️
src/_pytest/debugging.py	55.81% <0%> (-11.63%)	⬇️
src/_pytest/python_api.py	87.01% <0%> (-10.39%)	⬇️
testing/test_pdb.py	43.86% <0%> (-9.27%)	⬇️
src/_pytest/assertion/util.py	92.92% <0%> (-5.19%)	⬇️
src/_pytest/doctest.py	94.37% <0%> (-2.41%)	⬇️
... and 10 more

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[coveralls]

Coverage increased (+0.02%) to 93.825% when pulling 7a6d16c on jeffreyrack:4051-improve-error-messaging into d8d7f73 on pytest-dev:master.

[RonnyPfannschmidt]

failure is unrelated and will be fixed by @nicoddemus in another pr
great work on tackling this ripe fruit, thanks 👍