gbuild: print a better error message when adding non-existing Module

A confusing message "Corrupted module target stack!" is printed
because:

"If an included makefile cannot be found in any of these directories, a
 warning message is generated, but it is not an immediately fatal error;
 processing of the makefile containing the include continues.  Once it
 has finished reading makefiles, make will try to remake any that are
 out of date or don’t exist."

Change-Id: Ia728c0283885fe839dbf8dd8ae2a885230f23836
Reviewed-on: https://gerrit.libreoffice.org/47701Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>

solenv/gbuild/Module.mk

@@ -380,8 +380,13 @@ $(call gb_Module_get_clean_target,$(1)) : $$(gb_Module_CURRENTCLEANTARGET)
 
 endef
 
+define gb_Module__modulefile
+$(patsubst $(1):%,%,$(filter $(1):%,$(gb_Module_MODULELOCATIONS)))/$(2)/Module_$(2).mk
+endef
+
 define gb_Module_add_moduledir
-include $(patsubst $(1):%,%,$(filter $(1):%,$(gb_Module_MODULELOCATIONS)))/$(2)/Module_$(2).mk
+$(if $(wildcard $(call gb_Module__modulefile,$(1),$(2))),,$(call gb_Output_error,Module does not exist: $(call gb_Module__modulefile,$(1),$(2))))
+include $(call gb_Module__modulefile,$(1),$(2))
 $(call gb_Module_get_target,$(1)) : $$(firstword $$(gb_Module_TARGETSTACK))
 $(call gb_Module_get_l10n_target,$(1)) : $$(firstword $$(gb_Module_L10NTARGETSTACK))
 $(call gb_Module_get_check_target,$(1)) : $$(firstword $$(gb_Module_CHECKTARGETSTACK))