helpful error message when multiprocessing dir env is unset (#206)

On head it fails with:
ERROR in app: Exception on /metrics [GET]
AttributeError: 'NoneType' object has no attribute 'endswith'

With this change:
  File "build/bdist.linux-x86_64/egg/prometheus_client/multiprocess.py", line 16, in __init__
    raise ValueError('env prometheus_multiproc_dir is not set or not accessible')
ValueError: env prometheus_multiproc_dir is not set or not accessible

Author: dbalan

prometheus_client/multiprocess.py

@@ -12,6 +12,8 @@
 class MultiProcessCollector(object):
     """Collector for files for multi-process mode."""
     def __init__(self, registry, path=os.environ.get('prometheus_multiproc_dir')):
+        if not path or not os.path.isdir(path):
+            raise ValueError('env prometheus_multiproc_dir is not set or not a directory')
         self._path = path
         if registry:
           registry.register(self)

tests/test_multiprocess.py

@@ -158,3 +158,28 @@ def test_multi_expansion(self):
 
     def tearDown(self):
         os.unlink(self.tempfile)
+
+class TestUnsetEnv(unittest.TestCase):
+    def setUp(self):
+        self.registry = CollectorRegistry()
+        fp, self.tmpfl = tempfile.mkstemp()
+        os.close(fp)
+
+    def test_unset_syncdir_env(self):
+        self.assertRaises(
+            ValueError,
+            MultiProcessCollector,
+            self.registry
+        )
+
+    def test_file_syncpath(self):
+        registry = CollectorRegistry()
+        self.assertRaises(
+            ValueError,
+            MultiProcessCollector,
+            registry,
+            self.tmpfl
+        )
+
+    def tearDown(self):
+        os.remove(self.tmpfl)