Downloading files crashes if don't already have storage permission

[domdomegg]

TODO at bd97b9b#diff-0207cfb2164b044932178fb05fe3a752R185

Probably not caught as most users will upload a photo through the app before downloading

03-05 17:51:45.531 5505-10826/? E/ACRA: fr.free.nrw.commons fatal error : No permission to write to /storage/emulated/0/Download/Electric Bus on route 98 in London 08.jpg: Neither user 10357 nor current process has android.permission.WRITE_EXTERNAL_STORAGE.
                                        java.lang.SecurityException: No permission to write to /storage/emulated/0/Download/Electric Bus on route 98 in London 08.jpg: Neither user 10357 nor current process has android.permission.WRITE_EXTERNAL_STORAGE.
                                            at android.os.Parcel.readException(Parcel.java:1620)
                                            at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:183)
                                            at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:135)
                                            at android.content.ContentProviderProxy.insert(ContentProviderNative.java:476)
                                            at android.content.ContentResolver.insert(ContentResolver.java:1284)
                                            at android.app.DownloadManager.enqueue(DownloadManager.java:1336)
                                            at fr.free.nrw.commons.media.MediaDetailPagerFragment.downloadMedia(MediaDetailPagerFragment.java:186)
                                            at fr.free.nrw.commons.media.MediaDetailPagerFragment.onOptionsItemSelected(MediaDetailPagerFragment.java:145)
                                            at android.support.v4.app.Fragment.performOptionsItemSelected(Fragment.java:2212)
                                            at android.support.v4.app.FragmentManagerImpl.dispatchOptionsItemSelected(FragmentManager.java:2295)
                                            at android.support.v4.app.FragmentController.dispatchOptionsItemSelected(FragmentController.java:353)
                                            at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:414)
                                            at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:198)
                                            at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:107)
                                            at android.support.v7.app.AppCompatDelegateImplV9.onMenuItemSelected(AppCompatDelegateImplV9.java:671)
                                            at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:817)
                                            at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:156)
                                            at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:964)
                                            at android.support.v7.view.menu.MenuPopup.onItemClick(MenuPopup.java:127)
                                            at android.widget.AdapterView.performItemClick(AdapterView.java:345)
                                            at android.widget.AbsListView.performItemClick(AbsListView.java:1547)
                                            at android.widget.AbsListView$PerformClick.run(AbsListView.java:3821)
                                            at android.widget.AbsListView$3.run(AbsListView.java:5843)
                                            at android.os.Handler.handleCallback(Handler.java:739)
                                            at android.os.Handler.dispatchMessage(Handler.java:95)
                                            at android.os.Looper.loop(Looper.java:158)
                                            at android.app.ActivityThread.main(ActivityThread.java:7229)
                                            at java.lang.reflect.Method.invoke(Native Method)
                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)