Provide better error for nonexistent link target

UX enhancement regarding #580.

Previously MacDown delegates error-handling to Finder when a link
to nonexistent local file is clicked in preview (via NSWorkspace).
This leads to a generic "Can't find the file XXX" message, which
is not really useful.

This patch detects a file URL in the above scenario, and shows a
better message inside MacDown that provides an absolute path
(instead of just the filename), and hints the user to use the
auto-create feature if desired.

Behaviours are unchanged for cases which...

* The URL is not a local file.
* The target file exists.
* The auto-create option is on.

MacDown/Code/Document/MPDocument.m

@@ -826,6 +826,7 @@ - (void)webView:(WebView *)webView
         request:(NSURLRequest *)request frame:(WebFrame *)frame
                 decisionListener:(id<WebPolicyDecisionListener>)listener
 {
+//    NSURL *url = information[WebActionOriginalURLKey];
     switch ([information[WebActionNavigationTypeKey] integerValue])
     {
         case WebNavigationTypeLinkClicked:
@@ -1595,14 +1596,29 @@ - (BOOL)isCurrentBaseUrl:(NSURL *)another
 
 - (void)openOrCreateFileForUrl:(NSURL *)url
 {
-    // TODO: Make this togglable in preferences.
-    // If this is a file URL and the target does not exist, create and open it.
-    if (self.preferences.createFileForLinkTarget && url.isFileURL
-        && ![[NSFileManager defaultManager] fileExistsAtPath:url.path])
+    // If the URL points to a nonexistent file, create automatically if
+    // requested, or provide better error message.
+    if (url.isFileURL && ![url checkResourceIsReachableAndReturnError:NULL])
     {
-        NSDocumentController *controller =
-            [NSDocumentController sharedDocumentController];
-        [controller openUntitledDocumentForURL:url display:YES error:NULL];
+        if (self.preferences.createFileForLinkTarget)
+        {
+            NSDocumentController *controller =
+                [NSDocumentController sharedDocumentController];
+            [controller openUntitledDocumentForURL:url display:YES error:NULL];
+            return;
+        }
+
+        NSAlert *alert = [[NSAlert alloc] init];
+        NSString *template = NSLocalizedString(
+            @"File not found at path:\n%@",
+            @"preview navigation error message");
+        alert.messageText = [NSString stringWithFormat:template, url.path];
+        alert.informativeText = NSLocalizedString(
+            @"Please check the path of your link is correct. Turn on "
+            @"“Automatically create link targets” If you want MacDown to "
+            @"create nonexistent link targets for you",
+            @"preview navigation error information");
+        [alert runModal];
         return;
     }