Use a nice string repr for ConftestImportFailure

The default message is often hard to read:

    E   _pytest.config.ConftestImportFailure: (local('D:\\projects\\pytest\\.tmp\\root\\foo\\conftest.py'), (<class 'RuntimeError'>, RuntimeError('some error',), <traceback object at 0x000001CCC3E39348>))

Using a shorter message is better:

    E   _pytest.config.ConftestImportFailure: RuntimeError: some error (from D:\projects\pytest\.tmp\root\foo\conftest.py)

And we don't really lose any information due to exception chaining.

src/_pytest/config/__init__.py

@@ -88,10 +88,15 @@ class ExitCode(enum.IntEnum):
 
 class ConftestImportFailure(Exception):
     def __init__(self, path, excinfo):
-        Exception.__init__(self, path, excinfo)
+        super().__init__(path, excinfo)
         self.path = path
         self.excinfo = excinfo  # type: Tuple[Type[Exception], Exception, TracebackType]
 
+    def __str__(self):
+        return "{}: {} (from {})".format(
+            self.excinfo[0].__name__, self.excinfo[1], self.path
+        )
+
 
 def main(args=None, plugins=None) -> Union[int, ExitCode]:
     """ return exit code, after performing an in-process test run.