Avoid crash after OSError during django path detection

[voidus]

Fixes #662

[codecov-io]

Codecov Report

Merging #664 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #664      +/-   ##
==========================================
+ Coverage   94.36%   94.38%   +0.01%     
==========================================
  Files          33       33              
  Lines        1828     1834       +6     
  Branches      157      156       -1     
==========================================
+ Hits         1725     1731       +6     
  Misses         78       78              
  Partials       25       25

Flag	Coverage Δ
#dj110	86.2% <100%> (+0.04%)	⬆️
#dj111	88.6% <100%> (+0.03%)	⬆️
#dj18	87.18% <100%> (+0.04%)	⬆️
#dj19	85.98% <100%> (+0.04%)	⬆️
#dj20	86.47% <100%> (+0.04%)	⬆️
#dj21	82.49% <100%> (+0.05%)	⬆️
#djmaster	82.49% <100%> (+0.05%)	⬆️
#mysql_innodb	86.69% <100%> (+0.04%)	⬆️
#mysql_myisam	86.53% <100%> (+0.04%)	⬆️
#postgres	89.85% <100%> (+0.03%)	⬆️
#py27	91.54% <100%> (+0.02%)	⬆️
#py34	85.98% <100%> (+0.04%)	⬆️
#py35	86.2% <100%> (+0.04%)	⬆️
#py36	87.4% <100%> (+0.04%)	⬆️
#py37	82.49% <100%> (+0.05%)	⬆️
#sqlite	88.54% <100%> (+0.03%)	⬆️
#sqlite_file	85.98% <100%> (+0.04%)	⬆️

Impacted Files	Coverage Δ
tests/test_manage_py_scan.py	100% <100%> (ø)	⬆️
pytest_django/plugin.py	93.62% <100%> (+0.03%)	⬆️

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[blueyed]

Thank you!