spirv: Improve the "Entry point not found" error message

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6607>

src/compiler/spirv/spirv_to_nir.c

@@ -5646,7 +5646,8 @@ spirv_to_nir(const uint32_t *words, size_t word_count,
                                    vtn_handle_preamble_instruction);
 
    if (b->entry_point == NULL) {
-      vtn_fail("Entry point not found");
+      vtn_fail("Entry point not found for %s shader \"%s\"",
+               _mesa_shader_stage_to_string(stage), entry_point_name);
       ralloc_free(b);
       return NULL;
    }