Update GrRenderTask execution error handling

GrDrawingManager flush() and executeRenderTasks() were mishandling the status returned from GrRenderTask execute/onExecute. This separates out the "has any task been run" and "has an error occurred" concerns to clarify things.

Bug: b/553122373
Change-Id: I8f98ffbf97f4598a136811760871f16f91d7067d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1344836
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Thomas Smith <thomsmit@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>