SH-4310/BUG-2810/MAINT-2794 Better status checking and error logging in Mesh code.

Pay correct attention to status codes coming back from services.  Generate
better and consistent error messages when problems arise.  There's more to
do in error handling, need a way to cleanly fail all request types, only
have that for LOD at this point.  Do better keeping the HTTP pipeline between
the low and high water marks.  This was made challenging because the outer
most code couldn't really see what's going on internally (whose actions are
delayed in a worker thread).  More to do here, the debug-like requests don't
honor limits, that will come later.  Made retry counts available from llcorehttp
which can be used by the throttle-anticipating logic to advance the count.
It helps but it reinforces the coupling between viewer and server which I
do not like.