More verbose error messages for iOS

Summary:
Output the reason for the error when failing to load source code. This was a big help when trying to diagnose #13299.

~~Unfortunately there still seems to be no way to get the offending line number (because `loadError.userInfo[RCTJSStackTraceKey]` is empty), but this is good enough.~~

Before:

```
[warn][tid:com.facebook.react.JavaScript][RCTBatchedBridge.m:510] Failed to execute source code. Unhandled JS Exception: SyntaxError
```

After:

```
[warn][tid:com.facebook.react.JavaScript][RCTJSCErrorHandling.mm:30] Couldn't get stack trace for http://localhost:8081/index.ios.bundle?platform=ios&dev=true&minify=false:81886
[warn][tid:com.facebook.react.JavaScript][RCTBatchedBridge.m:510] Failed to execute source code. Unhandled JS Exception: SyntaxError Unexpected keyword 'var'
```
Closes #13561

Differential Revision: D4908501

Pulled By: javache

fbshipit-source-id: a316dc70739b917b3cc690309d0ff37a8bb5d412

React/Base/RCTBatchedBridge.m

@@ -210,7 +210,7 @@ - (void)loadSource:(RCTSourceLoadBlock)_onSourceLoad onProgress:(RCTSourceLoadPr
       if (error && [self.delegate respondsToSelector:@selector(fallbackSourceURLForBridge:)]) {
         NSURL *fallbackURL = [self.delegate fallbackSourceURLForBridge:self->_parentBridge];
         if (fallbackURL && ![fallbackURL isEqual:self.bundleURL]) {
-          RCTLogError(@"Failed to load bundle(%@) with error:(%@)", self.bundleURL, error.localizedDescription);
+          RCTLogError(@"Failed to load bundle(%@) with error:(%@ %@)", self.bundleURL, error.localizedDescription, error.localizedFailureReason);
           self.bundleURL = fallbackURL;
           [RCTJavaScriptLoader loadBundleAtURL:self.bundleURL onProgress:onProgress onComplete:onSourceLoad];
           return;
@@ -513,7 +513,7 @@ - (void)executeSourceCode:(NSData *)sourceCode
     }
 
     if (loadError) {
-      RCTLogWarn(@"Failed to execute source code: %@", [loadError localizedDescription]);
+      RCTLogWarn(@"Failed to execute source code: %@ %@", [loadError localizedDescription], [loadError localizedFailureReason]);
       dispatch_async(dispatch_get_main_queue(), ^{
         [self stopLoadingWithError:loadError];
       });

React/Base/RCTJSCErrorHandling.mm

@@ -13,6 +13,7 @@
 
 #import "RCTAssert.h"
 #import "RCTJSStackFrame.h"
+#import "RCTLog.h"
 
 NSString *const RCTJSExceptionUnsymbolicatedStackTraceKey = @"RCTJSExceptionUnsymbolicatedStackTraceKey";
 
@@ -25,7 +26,9 @@
     userInfo[NSLocalizedFailureReasonErrorKey] = exceptionMessage;
   }
   NSString *const stack = [exception[@"stack"] toString];
-  if ([stack length]) {
+  if ([@"undefined" isEqualToString:stack]) {
+    RCTLogWarn(@"Couldn't get stack trace for %@:%@", exception[@"sourceURL"], exception[@"line"]);
+  } else if ([stack length]) {
     NSArray<RCTJSStackFrame *> *const unsymbolicatedFrames = [RCTJSStackFrame stackFramesWithLines:stack];
     userInfo[RCTJSStackTraceKey] = unsymbolicatedFrames;
   }

React/CxxBridge/RCTCxxBridge.mm

@@ -356,7 +356,7 @@ - (void)loadSource:(RCTSourceLoadBlock)_onSourceLoad onProgress:(RCTSourceLoadPr
       if (error && [self.delegate respondsToSelector:@selector(fallbackSourceURLForBridge:)]) {
         NSURL *fallbackURL = [self.delegate fallbackSourceURLForBridge:self->_parentBridge];
         if (fallbackURL && ![fallbackURL isEqual:self.bundleURL]) {
-          RCTLogError(@"Failed to load bundle(%@) with error:(%@)", self.bundleURL, error.localizedDescription);
+          RCTLogError(@"Failed to load bundle(%@) with error:(%@ %@)", self.bundleURL, error.localizedDescription, error.localizedFailureReason);
           self.bundleURL = fallbackURL;
           [RCTJavaScriptLoader loadBundleAtURL:self.bundleURL onProgress:onProgress onComplete:onSourceLoad];
           return;