Add some extra debug info when reading file contents fails

In response to #31

(To use the "as" syntax, changed run_py_tests.sh to use python2.7)
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=98748484

closure/bin/build/source.py

@@ -120,8 +120,13 @@ def GetFileContents(path):
     IOError: An error occurred opening or reading the file.
 
   """
-  fileobj = open(path)
+  fileobj = None
   try:
+    fileobj = open(path)
     return fileobj.read()
+  except IOError as error:
+    raise IOError('An error occurred opening or reading the file: %s. %s'
+                  % (path, error))
   finally:
-    fileobj.close()
+    if fileobj is not None:
+      fileobj.close()