Exit pytest on collection error (without executing tests)

Add --continue-on-collection-errors option to restore the previous behaviour:
Execute tests (that were successfully collected) even when collection errors
happen.

Some tests had to be modified e.g. because the return code changed to 2
(EXIT_INTERRUPTED) instead of 1 (EXIT_TESTSFAILED) because an Interrupted
exception is raised on collection error.

Implemented via pair programming with:
    Oleg Pidsadnyi <oleg.pidsadnyi@gmail.com>

closes #1421

_pytest/main.py

@@ -48,6 +48,9 @@ def pytest_addoption(parser):
                help="run pytest in strict mode, warnings become errors.")
     group._addoption("-c", metavar="file", type=str, dest="inifilename",
                help="load configuration from `file` instead of trying to locate one of the implicit configuration files.")
+    group._addoption("--continue-on-collection-errors", action="store_true",
+               default=False, dest="continue_on_collection_errors",
+               help="Force test execution even if collection errors occur.")
 
     group = parser.getgroup("collect", "collection")
     group.addoption('--collectonly', '--collect-only', action="store_true",
@@ -133,6 +136,11 @@ def pytest_collection(session):
     return session.perform_collect()
 
 def pytest_runtestloop(session):
+    if (session.testsfailed and
+            not session.config.option.continue_on_collection_errors):
+        raise session.Interrupted(
+            "%d errors during collection" % session.testsfailed)
+
     if session.config.option.collectonly:
         return True