fn: agent/lb/runner error handling adjustments (#1214)

1) Early call validation and return due to cpu/mem impossible
to meet (eg. request cpu/mem larger than max-mem or max-cpu
on server) now emits HTTP Bad Request (400) instead of 503.
This case is most likely due to client/service configuration
and/or validation issue.
2) 'failed' metric is now removed. 'failed' versus 'errors'
were too confusing. 'errors' is now a catch all error case.
3) new 'canceled' counter for client side cancels.
4) 'server_busy' now covers more cases than it previously did.

api/agent/call.go

@@ -428,8 +428,7 @@ func (a *agent) GetCall(opts ...CallOpt) (Call, error) {
 
 	mem := c.Memory + uint64(c.TmpFsSize)
 	if !a.resources.IsResourcePossible(mem, uint64(c.CPUs), c.Type == models.TypeAsync) {
-		// if we're not going to be able to run this call on this machine, bail here.
-		return nil, models.ErrCallTimeoutServerBusy
+		return nil, models.ErrCallResourceTooBig
 	}
 
 	setupCtx(&c)