Migrate to using ReportError to report a failure to evaluate a
watchpoint condition. I had already done so for the parallel code for
breakpoints.
In the process, I noticed that I accidentally regressed the error
reporting for breakpoint conditions by dropping the call to
GetDescription. This patch rectifies that and adds a test.
Because the call to GetDescription expects a Stream*, I also switches
from using a raw_string_ostream to a StreamString for both breakpoints
and watchpoints.
