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/agent_test.go

@@ -581,7 +581,7 @@ func TestGetCallReturnsResourceImpossibility(t *testing.T) {
 	defer checkClose(t, a)
 
 	_, err := a.GetCall(FromModel(call))
-	if err != models.ErrCallTimeoutServerBusy {
+	if err != models.ErrCallResourceTooBig {
 		t.Fatal("did not get expected err, got: ", err)
 	}
 }