Clarify genrule error message

Point the user toward checking their $(location) label is mentioned
elsewhere in the build rule (as opposed to a typo of the module name,
for example).

Test: genrule_test.go
Change-Id: Icc19740938e2b56eef24540534f9cc5bfa9420b8
diff --git a/genrule/genrule.go b/genrule/genrule.go
index 8282426..2ddd70e 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -496,7 +496,7 @@
 						}
 						return paths[0], nil
 					} else {
-						return reportError("unknown location label %q", label)
+						return reportError("unknown location label %q is not in srcs, out, tools or tool_files.", label)
 					}
 				} else if strings.HasPrefix(name, "locations ") {
 					label := strings.TrimSpace(strings.TrimPrefix(name, "locations "))
@@ -507,7 +507,7 @@
 						}
 						return strings.Join(paths, " "), nil
 					} else {
-						return reportError("unknown locations label %q", label)
+						return reportError("unknown locations label %q is not in srcs, out, tools or tool_files.", label)
 					}
 				} else {
 					return reportError("unknown variable '$(%s)'", name)