builder: add a verror, when the output folder is not existing and writable (#5966)

vlib/v/builder/compile.v

@@ -27,6 +27,18 @@ fn get_vtmp_filename(base_file_name, postfix string) string {
 }
 
 pub fn compile(command string, pref &pref.Preferences) {
+	odir := os.base_dir(pref.out_name)
+	// When pref.out_name is just the name of an executable, i.e. `./v -o executable main.v`
+	// without a folder component, just use the current folder instead:
+	mut output_folder := odir
+	if odir.len == pref.out_name.len {
+		output_folder = os.getwd()
+	}        
+	os.is_writable_folder(output_folder) or { 
+		// An early error here, is better than an unclear C error later:
+		verror(err)
+		exit(1)
+	}
 	// Construct the V object from command line arguments
 	mut b := new_builder(pref)
 	if pref.is_verbose {