Improve error message when uploading attachment to Pixelfed fails

See #425

Packages/Pachyderm/Sources/Pachyderm/Client.swift

@@ -113,6 +113,7 @@ public class Client {
         var urlRequest = URLRequest(url: url, timeoutInterval: timeoutInterval)
         urlRequest.httpMethod = request.method.name
         urlRequest.httpBody = request.body.data
+        urlRequest.setValue("application/json", forHTTPHeaderField: "Accept")
         for (name, value) in request.headers {
             urlRequest.setValue(value, forHTTPHeaderField: name)
         }

Packages/Pachyderm/Sources/Pachyderm/Model/MastodonError.swift

@@ -11,7 +11,20 @@ import Foundation
 struct MastodonError: Decodable, CustomStringConvertible {
     var description: String
     
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        if let error = try container.decodeIfPresent(String.self, forKey: .error) {
+            self.description = error
+        } else if let message = try container.decodeIfPresent(String.self, forKey: .message) {
+            self.description = message
+        } else {
+            throw DecodingError.keyNotFound(CodingKeys.error, .init(codingPath: container.codingPath, debugDescription: "Missing error or message key"))
+        }
+    }
+    
     private enum CodingKeys: String, CodingKey {
-        case description = "error"
+        case error
+        // used by pixelfed
+        case message
     }
 }