Provide more human readable error when downloading to invalid directory (#2462)

(cherry picked from commit 13a2d3dfdd)

app/src/main/java/eu/kanade/tachiyomi/data/download/DownloadProvider.kt

@@ -3,6 +3,7 @@ package eu.kanade.tachiyomi.data.download
 import android.content.Context
 import android.net.Uri
 import com.hippo.unifile.UniFile
+import eu.kanade.tachiyomi.R
 import eu.kanade.tachiyomi.data.database.models.Chapter
 import eu.kanade.tachiyomi.data.database.models.Manga
 import eu.kanade.tachiyomi.data.preference.PreferencesHelper
@@ -46,9 +47,13 @@ class DownloadProvider(private val context: Context) {
      * @param source the source of the manga.
      */
     internal fun getMangaDir(manga: Manga, source: Source): UniFile {
-        return downloadsDir
-                .createDirectory(getSourceDirName(source))
-                .createDirectory(getMangaDirName(manga))
+        try {
+            return downloadsDir
+                    .createDirectory(getSourceDirName(source))
+                    .createDirectory(getMangaDirName(manga))
+        } catch (e: NullPointerException) {
+            throw Exception(context.getString(R.string.invalid_download_dir))
+        }
     }
 
     /**

app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt

@@ -102,7 +102,7 @@ class Downloader(
         pending.forEach { if (it.status != Download.QUEUE) it.status = Download.QUEUE }
 
         downloadsRelay.call(pending)
-        return !pending.isEmpty()
+        return pending.isNotEmpty()
     }
 
     /**

app/src/main/res/values/strings.xml

@@ -393,6 +393,7 @@
     <string name="download_all">Download all</string>
     <string name="download_unread">Download unread</string>
     <string name="confirm_delete_chapters">Are you sure you want to delete selected chapters?</string>
+    <string name="invalid_download_dir">Invalid download directory</string>
 
     <!-- Tracking Screen -->
     <string name="manga_tracking_tab">Tracking</string>