Don't ignore ImportError with setuptools plugins

This was added in b2d66b9 but is a bad
idea. When a plugin can't be imported, commandline options (optionally
set in pytest.ini) could be undefined, which means pytest bails out
much earlier before showing the warning, which is hard to debug.

Fixes pytest-dev#1479, also see pytest-dev#1307 and pytest-dev#1497

_pytest/config.py

@@ -923,10 +923,7 @@ def _preparse(self, args, addopts=True):
             args[:] = self.getini("addopts") + args
         self._checkversion()
         self.pluginmanager.consider_preparse(args)
-        try:
-            self.pluginmanager.load_setuptools_entrypoints("pytest11")
-        except ImportError as e:
-            self.warn("I2", "could not load setuptools entry import: %s" % (e,))
+        self.pluginmanager.load_setuptools_entrypoints("pytest11")
         self.pluginmanager.consider_env()
         self.known_args_namespace = ns = self._parser.parse_known_args(args, namespace=self.option.copy())
         if self.known_args_namespace.confcutdir is None and self.inifile:

testing/test_config.py

@@ -391,6 +391,23 @@ class PseudoPlugin:
     plugin = config.pluginmanager.getplugin("mytestplugin")
     assert plugin.x == 42
 
+
+def test_setuptools_importerror_issue1479(testdir, monkeypatch):
+    pkg_resources = pytest.importorskip("pkg_resources")
+    def my_iter(name):
+        assert name == "pytest11"
+        class EntryPoint:
+            name = "mytestplugin"
+            dist = None
+            def load(self):
+                raise ImportError("Don't hide me!")
+        return iter([EntryPoint()])
+
+    monkeypatch.setattr(pkg_resources, 'iter_entry_points', my_iter)
+    with pytest.raises(ImportError):
+        testdir.parseconfig()
+
+
 def test_plugin_preparse_prevents_setuptools_loading(testdir, monkeypatch):
     pkg_resources = pytest.importorskip("pkg_resources")
     def my_iter(name):