Begin mapping os.Error in the rest of the codebase

tests/core/flags/test_core_flags.odin

@@ -1117,7 +1117,7 @@ test_os_handle :: proc(t: ^testing.T) {
 		// Delete the file now that we're done.
 		//
 		// This is not done all the time, just in case the file is useful to debugging.
-		testing.expect_value(t, os.remove(TEMPORARY_FILENAME), os.ERROR_NONE)
+		testing.expect_value(t, os.remove(TEMPORARY_FILENAME), nil)
 	}
 
 	TEMPORARY_FILENAME :: "test_core_flags_write_test_output_data"

tests/core/os/os.odin

@@ -7,12 +7,12 @@ import "core:testing"
 
 @(test)
 read_dir :: proc(t: ^testing.T) {
-	fd, errno := os.open(#directory + "/dir")
-	testing.expect_value(t, errno, os.ERROR_NONE)
+	fd, err := os.open(#directory + "/dir")
+	testing.expect_value(t, err, nil)
 	defer os.close(fd)
 
-	dir, errno2 := os.read_dir(fd, -1)
-	testing.expect_value(t, errno2, os.ERROR_NONE)
+	dir, err2 := os.read_dir(fd, -1)
+	testing.expect_value(t, err2, nil)
 	defer os.file_info_slice_delete(dir)
 
 	slice.sort_by_key(dir, proc(fi: os.File_Info) -> string { return fi.name })

tests/documentation/documentation_tester.odin

@@ -439,7 +439,7 @@ main :: proc() {
 		}
 		save_path := fmt.tprintf("verify/test_%v_%v.odin", test.package_name, code_test_name)
 
-		test_file_handle, err := os.open(save_path, os.O_WRONLY | os.O_CREATE); if err != 0 {
+		test_file_handle, err := os.open(save_path, os.O_WRONLY | os.O_CREATE); if err != nil {
 			fmt.eprintf("We could not open the file to the path %q for writing\n", save_path)
 			g_bad_doc = true
 			continue