Polished download series function

- File and size resets every time when download is triggered
- If everything is downloaded show popup to tell it to user

pages/bookshelf/series/_id.vue

@@ -126,6 +126,8 @@ export default {
       // Fetch series data from server
       let page = 0
       let fetchFinished = false
+      this.missingFiles = 0
+      this.missingFilesSize = 0
       while (fetchFinished === false) {
         fetchFinished = await this.fetchSeriesEntities(page)
         page += 1
@@ -134,6 +136,9 @@ export default {
         console.error('failed to fetch series books data')
         return null
       }
+      if (this.missingFiles == 0) {
+        alert(this.$getString('MessageSeriesAlreadyDownloaded'))
+      }
 
       // Format message for dialog
       let startDownloadMessage = this.$getString('MessageSeriesDownloadConfirmIos', [this.libraryIds.length, this.missingFiles, this.$bytesPretty(this.missingFilesSize)])