Improve error format with errors during setup

[pytestbot]

Originally reported by: Daniel Hahler (BitBucket: blueyed, GitHub: blueyed)

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When there is an error during setup (in case of a missing fixture), the
output is not easily parsable:

_____________ ERROR at setup of test_foo ______________
file …/project/app/test_models.py, line 334
  def test_foo(db2):
        fixture 'db2' not found
        available fixtures: _django_clear_outbox, ...
        use 'py.test --fixtures [testpath]' for help on them.

…/project/app/test_models.py:334

It would be nice if the line containing the error ("fixture 'db2' not found")
was prefixed with "E", and maybe the additional information with ">".

I am in the process of improving the errorformat string for Vim, defined/based
on the pytest-vim-compiler
plugin.

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• Bitbucket: https://bitbucket.org/pytest-dev/pytest/issue/717