Invoke completion block on main dispatch queue.

Also improve error description.

Mobile Buy SDK/Mobile Buy SDK/Models/BUYError.m

@@ -32,7 +32,7 @@ NSString * const BUYShopifyError = @"BUYShopifyError";
 
 - (NSString *)description
 {
-	return [[self userInfo] description];
+	return [NSString stringWithFormat:@"Error code %td: %@", self.code, [self userInfo]];
 }
 
 @end

Mobile Buy SDK/Mobile Buy SDK/Utils/BUYApplePayHelpers.m

@@ -331,8 +331,10 @@ const NSTimeInterval PollDelay = 0.5;
 				[NSThread sleepForTimeInterval:PollDelay];
 			}
 		}
+		dispatch_async(dispatch_get_main_queue(), ^{
 			completion(checkoutStatus == BUYStatusComplete ? PKPaymentAuthorizationStatusSuccess : PKPaymentAuthorizationStatusFailure);
 		});
+	});
 }
 
 @end