Clarify OWSOperation behavior (no behavioral changes)

SignalServiceKit/src/Util/OWSOperation.h

@@ -24,22 +24,26 @@ typedef NS_ENUM(NSInteger, OWSOperationState) {
 // any of the errors were fatal. Fatal errors trump retryable errors.
 @interface OWSOperation : NSOperation
 
-@property (nullable) NSError *failingError;
+@property (readonly, nullable) NSError *failingError;
-@property NSUInteger remainingRetries;
 
-#pragma mark - Subclass Overrides
+// Defaults to 0, set to greater than 0 in init if you'd like the operation to be retryable.
+@property NSUInteger remainingRetries;
 
-// Called one time only
+#pragma mark - Mandatory Subclass Overrides
-- (nullable NSError *)checkForPreconditionError;
 
 // Called every retry, this is where the bulk of the operation's work should go.
 - (void)run;
 
+#pragma mark - Optional Subclass Overrides
+
+// Called one time only
+- (nullable NSError *)checkForPreconditionError;
+
 // Called at most one time.
 - (void)didSucceed;
 
 // Called at most one time, once retry is no longer possible.
-- (void)didFailWithError:(NSError *)error;
+- (void)didFailWithError:(NSError *)error NS_SWIFT_NAME(didFail(error:));
 
 #pragma mark - Success/Error - Do Not Override
 

SignalServiceKit/src/Util/OWSOperation.m

@@ -13,6 +13,7 @@ NSString *const OWSOperationKeyIsFinished = @"isFinished";
 
 @interface OWSOperation ()
 
+@property (nullable) NSError *failingError;
 @property (atomic) OWSOperationState operationState;
 @property (nonatomic) OWSBackgroundTask *backgroundTask;