StringDownload step: more informative error message

Changed error message when buildbot.steps.transfer.StringDownload
is instantiated with invalid 'mode' argument. Include more context
on which 'mode' is invalid.

master/buildbot/steps/transfer.py

@@ -481,7 +481,8 @@ def __init__(self, s, slavedest,
         self.blocksize = blocksize
         if not isinstance(mode, (int, type(None))):
             config.error(
-                'mode must be an integer or None')
+                "StringDownload step's mode must be an integer or None,"
+                " got '%s'" % mode)
         self.mode = mode
 
     def start(self):

master/buildbot/test/unit/test_steps_transfer.py

@@ -183,6 +183,16 @@ def upload_behavior(command):
         return d
 
 class TestStringDownload(unittest.TestCase):
+
+    # check that ConfigErrors is raised on invalid 'mode' argument
+    def testModeConfError(self):
+        self.assertRaisesRegexp(
+            config.ConfigErrors,
+            "StringDownload step's mode must be an integer or None,"
+            " got 'not-a-number'",
+            transfer.StringDownload,
+            "string", "file", mode="not-a-number")
+
     def testBasic(self):
         s = transfer.StringDownload("Hello World", "hello.txt")
         s.build = Mock()