grabber: timeout fixes

RELEASE_NOTES.rst

@@ -46,7 +46,7 @@ Highlights:
 * MSYS2 packaging for Windows, which replaces Conda. Conda packages are still available to
   support legacy installations, but may be removed in a future release.
 * Experiments can now be submitted with revisions set to a branch / tag name instead of only git hashes.
-* Grabber image input now has an optional timeout
+* Grabber image input now has an optional timeout.
 
 Breaking changes:
 

artiq/coredevice/grabber.py

@@ -101,8 +101,8 @@ class Grabber:
         ROI engines that produced output, an exception will be raised during
         this call or the next.
         
-        If the timeout is reached before data is available, a RuntimeError is
-        raised.
+        If the timeout is reached before data is available, the exception
+        GrabberTimeoutException is raised.
 
         :param timeout_mu: Timestamp at which a timeout will occur. Set to -1
                            (default) to disable timeout. 
@@ -120,5 +120,6 @@ class Grabber:
             if roi_output == self.sentinel:
                 raise OutOfSyncException
             if timestamp == -1:
-                raise GrabberTimeoutException("Timeout retrieving ROIs")
+                raise GrabberTimeoutException(
+                    "Timeout retrieving ROIs (attempting to read more ROIs than enabled?)")
             data[i] = roi_output