Show traceback during collection

[nicoddemus]

Fixes #1976.

Here's the output:

master

______________________ ERROR collecting .tmp/test_foo.py ______________________
ImportError while importing test module 'C:\pytest\.tmp\test_foo.py'.
Original error message:
'cannot import name 'non_existing''
Make sure your test modules/packages have valid Python names.
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!

this PR

______________________ ERROR collecting .tmp/test_foo.py ______________________
ImportError while importing test module 'C:\pytest\.tmp\test_foo.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
test_foo.py:1: in <module>
    import foo
foo.py:1: in <module>
    from bar import non_existing
E   ImportError: cannot import name 'non_existing'
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!

this PR with -vv

______________________ ERROR collecting .tmp/test_foo.py ______________________
ImportError while importing test module 'C:\pytest\.tmp\test_foo.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
..\_pytest\python.py:415: in _importtestmodule
    mod = self.fspath.pyimport(ensuresyspath=importmode)
..\.env35\lib\site-packages\py\_path\local.py:650: in pyimport
    __import__(modname)
..\_pytest\assertion\rewrite.py:207: in load_module
    py.builtin.exec_(co, mod.__dict__)
test_foo.py:1: in <module>
    import foo
foo.py:1: in <module>
    from bar import non_existing
E   ImportError: cannot import name 'non_existing'
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!

[nicoddemus]

This test was here to explicitly cut back the traceback from the error message, but unfortunately this makes it more difficult to find bugs.

[nicoddemus]

Decided to hard-code it to short because it is usually enough to track import problems and long may generate too much output during collection.

[RonnyPfannschmidt]

please a comment in the code

[nicoddemus]

Ops, good point.

[coveralls]

Coverage decreased (-0.04%) to 92.909% when pulling a1d446b on nicoddemus:show-traceback-during-collection into fc02003 on pytest-dev:master.

[nicoddemus]

No idea why the py35-trial failed, will investigate it later.