fix error message

src/core/content/content.js

@@ -109,7 +109,7 @@ async function onMessage(message) {
 					});
 				} catch (error) {
 					return {
-						error: error.toString()
+						error: error && (error.message || error.toString())
 					};
 				} finally {
 					await browser.runtime.sendMessage({ method: "downloads.end", taskId: result.value.taskId });
@@ -176,7 +176,7 @@ async function savePage(message) {
 			} catch (error) {
 				if (!processor.cancelled) {
 					console.error(error); // eslint-disable-line no-console
-					const errorMessage = error.toString();
+					const errorMessage = error && (error.message || error.toString());
 					browser.runtime.sendMessage({ method: "ui.processError", error: errorMessage });
 					onError(errorMessage);
 				}

src/lib/single-file/fetch/bg/fetch.js

@@ -38,7 +38,7 @@ browser.runtime.onMessage.addListener((message, sender) => {
 		return new Promise(resolve => {
 			onRequest(message, sender)
 				.then(resolve)
-				.catch(error => resolve({ error: error && error.toString() }));
+				.catch(error => resolve({ error: error && (error.message || error.toString()) }));
 		});
 	}
 });

src/lib/single-file/fetch/content/content-fetch.js

@@ -55,7 +55,7 @@ async function onFetchFrame(message) {
 		};
 	} catch (error) {
 		return {
-			error: error && error.toString()
+			error: error && (error.message || error.toString())
 		};
 	}
 }