Ignore command line --build-solutions when not editing project

This option is meant to use together with `--path` or from a project
folder. Otherwise the project manager is opened and the option triggers
a crash.

Fixes #25589.

main/main.cpp

@@ -292,7 +292,7 @@ void Main::print_help(const char *p_binary) {
 	OS::get_singleton()->print("  --export-pack <preset> <path>    Same as --export, but only export the game pack for the given preset. The <path> extension determines whether it will be in PCK or ZIP format.\n");
 	OS::get_singleton()->print("  --doctool <path>                 Dump the engine API reference to the given <path> in XML format, merging if existing files are found.\n");
 	OS::get_singleton()->print("  --no-docbase                     Disallow dumping the base types (used with --doctool).\n");
-	OS::get_singleton()->print("  --build-solutions                Build the scripting solutions (e.g. for C# projects).\n");
+	OS::get_singleton()->print("  --build-solutions                Build the scripting solutions (e.g. for C# projects). Implies --editor and requires a valid project to edit.\n");
 #ifdef DEBUG_METHODS_ENABLED
 	OS::get_singleton()->print("  --gdnative-generate-json-api     Generate JSON dump of the Godot API for GDNative bindings.\n");
 #endif
@@ -2112,8 +2112,12 @@ bool Main::iteration() {
 #ifdef TOOLS_ENABLED
 	if (auto_build_solutions) {
 		auto_build_solutions = false;
+		// Only relevant when running the editor.
+		if (!editor) {
+			ERR_FAIL_V_MSG(true, "Command line option --build-solutions was passed, but no project is being edited. Aborting.");
+		}
 		if (!EditorNode::get_singleton()->call_build()) {
-			ERR_FAIL_V(true);
+			ERR_FAIL_V_MSG(true, "Command line option --build-solutions was passed, but the build callback failed. Aborting.");
 		}
 	}
 #endif