Add mixed-build args (system build info) in acloud create.

- Add system image args to support creating AVD with specific system
image.

Bug:138098262
Test: acloud-dev create --system-branch aosp-master

Change-Id: I67089a2d906ddfd85e0bb71037bef4928146ec5b
diff --git a/errors.py b/errors.py
index d145ce7..f2c027b 100644
--- a/errors.py
+++ b/errors.py
@@ -151,6 +151,10 @@
     """Unsupported create action for multi AVDs and specify adb port."""
 
 
+class UnsupportedCreateArgs(CreateError):
+    """Unsupported create arg for a specified AVD type."""
+
+
 class GetBuildIDError(CreateError):
     """Can't get build id from Android Build."""