Fix nullability warnings in RCTExceptionsManager (#24467)

Summary:
There are a number of nullability warnings in `RCTExceptionsManager` when building React Native for iOS with Xcode 10.2. This resolves them without changing the existing behavior.

See the warnings here:

![image](https://user-images.githubusercontent.com/1773641/56201323-89551380-6038-11e9-9998-b6a8d3d28f36.png)

[iOS] [Fixed] - Fix nullability warnings in RCTExceptionsManager
Pull Request resolved: #24467

Differential Revision: D14973485

Pulled By: cpojer

fbshipit-source-id: 2fac9f067ac9418397c3913760a2403f9a2cc147

Author: jtreanor

React/Modules/RCTExceptionsManager.h

@@ -9,25 +9,29 @@
 
 #import <React/RCTBridgeModule.h>
 
+NS_ASSUME_NONNULL_BEGIN
+
 @protocol RCTExceptionsManagerDelegate <NSObject>
 
-- (void)handleSoftJSExceptionWithMessage:(NSString *)message stack:(NSArray *)stack exceptionId:(NSNumber *)exceptionId;
-- (void)handleFatalJSExceptionWithMessage:(NSString *)message stack:(NSArray *)stack exceptionId:(NSNumber *)exceptionId;
+- (void)handleSoftJSExceptionWithMessage:(nullable NSString *)message stack:(nullable NSArray *)stack exceptionId:(NSNumber *)exceptionId;
+- (void)handleFatalJSExceptionWithMessage:(nullable NSString *)message stack:(nullable NSArray *)stack exceptionId:(NSNumber *)exceptionId;
 
 @optional
-- (void)updateJSExceptionWithMessage:(NSString *)message stack:(NSArray *)stack exceptionId:(NSNumber *)exceptionId;
+- (void)updateJSExceptionWithMessage:(nullable NSString *)message stack:(nullable NSArray *)stack exceptionId:(NSNumber *)exceptionId;
 
 @end
 
 @interface RCTExceptionsManager : NSObject <RCTBridgeModule>
 
 - (instancetype)initWithDelegate:(id<RCTExceptionsManagerDelegate>)delegate;
 
-- (void)reportSoftException:(NSString *)message stack:(NSArray<NSDictionary *> *)stack exceptionId:(nonnull NSNumber *)exceptionId;
-- (void)reportFatalException:(NSString *)message stack:(NSArray<NSDictionary *> *)stack exceptionId:(nonnull NSNumber *)exceptionId;
+- (void)reportSoftException:(nullable NSString *)message stack:(nullable NSArray<NSDictionary *> *)stack exceptionId:(NSNumber *)exceptionId;
+- (void)reportFatalException:(nullable NSString *)message stack:(nullable NSArray<NSDictionary *> *)stack exceptionId:(NSNumber *)exceptionId;
 
 @property (nonatomic, weak) id<RCTExceptionsManagerDelegate> delegate;
 
 @property (nonatomic, assign) NSUInteger maxReloadAttempts;
 
 @end
+
+NS_ASSUME_NONNULL_END