[autotest] Do not allow unicode control files.

Our system does not properly handle non-ascii characters.  Therefore,
rather than having the scheduler explode somewhere down the line, let's
just explicitly disallow control files that contain unicode characters.

BUG=chromium-os:39337
TEST=local run_suite

Change-Id: Id2dcee75087803f1bd4236872f75e71039ae034b
Reviewed-on: https://gerrit.chromium.org/gerrit/44401
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Scott Zawalski <scottz@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
diff --git a/client/common_lib/error.py b/client/common_lib/error.py
index d9854db..2432e2b 100644
--- a/client/common_lib/error.py
+++ b/client/common_lib/error.py
@@ -514,6 +514,11 @@
     pass
 
 
+class ControlFileMalformed(CrosDynamicSuiteException):
+    """Raised when an invalid control file is read."""
+    pass
+
+
 class AsynchronousBuildFailure(CrosDynamicSuiteException):
     """Raised when the dev server throws 500 while finishing staging of a build.
     """