Better error messages when exporting video fails

Tusker/Models/CompositionAttachmentData.swift

@@ -88,8 +88,13 @@ enum CompositionAttachmentData {
                 options.isNetworkAccessAllowed = true
                 options.version = .current
                 PHImageManager.default().requestExportSession(forVideo: asset, options: options, exportPreset: AVAssetExportPresetHighestQuality) { (exportSession, info) in
-                    guard let exportSession = exportSession else { fatalError("failed to create export session") }
+                    if let exportSession = exportSession {
                         CompositionAttachmentData.exportVideoData(session: exportSession, completion: completion)
+                    } else if let error = info?[PHImageErrorKey] as? Error {
+                        completion(.failure(.videoExport(error)))
+                    } else {
+                        completion(.failure(.noVideoExportSession))
+                    }
                 }
             } else {
                 fatalError("assetType must be either image or video")
@@ -97,7 +102,8 @@ enum CompositionAttachmentData {
         case let .video(url):
             let asset = AVURLAsset(url: url)
             guard let session = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetHighestQuality) else {
-                fatalError("failed to create export session")
+                completion(.failure(.noVideoExportSession))
+                return
             }
             CompositionAttachmentData.exportVideoData(session: session, completion: completion)
             
@@ -112,14 +118,14 @@ enum CompositionAttachmentData {
         session.outputURL = FileManager.default.temporaryDirectory.appendingPathComponent("exported_video_\(UUID())").appendingPathExtension("mp4")
         session.exportAsynchronously {
             guard session.status == .completed else {
-                completion(.failure(.export(session.error!)))
+                completion(.failure(.videoExport(session.error!)))
                 return
             }
             do {
                 let data = try Data(contentsOf: session.outputURL!)
                 completion(.success((data, "video/mp4")))
             } catch {
-                completion(.failure(.export(error)))
+                completion(.failure(.videoExport(error)))
             }
         }
     }
@@ -128,9 +134,21 @@ enum CompositionAttachmentData {
         case image, video
     }
     
-    enum Error: Swift.Error {
+    enum Error: Swift.Error, LocalizedError {
         case missingData
-        case export(Swift.Error)
+        case videoExport(Swift.Error)
+        case noVideoExportSession
+        
+        var localizedDescription: String {
+            switch self {
+            case .missingData:
+                return "Missing Data"
+            case .videoExport(let error):
+                return "Exporting video: \(error)"
+            case .noVideoExportSession:
+                return "Couldn't create video export session"
+            }
+        }
     }
 }
 

Tusker/Screens/Compose/ComposeView.swift

@@ -350,6 +350,8 @@ fileprivate struct AttachmentUploadError: PostError {
             // need to downcast to use more specific localizedDescription impl from Pachyderm
             if let error = error as? Client.Error {
                 description = error.localizedDescription
+            } else if let error = error as? CompositionAttachmentData.Error {
+                description = error.localizedDescription
             } else {
                 description = error.localizedDescription
             }