extract convenience intitializer for param parser

SignalServiceKit/src/Contacts/OWSContactDiscoveryOperation.swift

@@ -390,12 +390,10 @@ class CDSBatchOperation: OWSOperation {
 
     func parseAndDecrypt(response: Any?, remoteAttestation: RemoteAttestation) throws -> Data {
 
-        guard let responseDict = response as? [String: AnyObject] else {
+        guard let params = ParamParser(responseObject: response) else {
             throw ContactDiscoveryError.parseError(description: "missing response dict")
         }
 
-        let params = ParamParser(dictionary: responseDict)
-
         let cipherText = try params.requiredBase64EncodedData(key: "data")
         let initializationVector = try params.requiredBase64EncodedData(key: "iv")
         let authTag = try params.requiredBase64EncodedData(key: "mac")

SignalServiceKit/src/Util/ParamParser.swift

@@ -34,6 +34,14 @@ public class ParamParser {
         self.dictionary = dictionary
     }
 
+    public convenience init?(responseObject: Any?) {
+        guard let responseDict = responseObject as? [String: AnyObject] else {
+            return nil
+        }
+
+        self.init(dictionary: responseDict)
+    }
+
     // MARK: Errors
 
     public enum ParseError: Error {