Add more information to __fbBatchedBridge is undefined error

Summary:
**Motivation:**  This error can be a symptom of various other issues (see: an issue search for `__fbBatchedBridge is undefined`). I'm hoping to provide slightly more information about what might be going wrong and how to self-help.

**Test Plan:** Run some JS before the bridge has injected itself into the JS context. (sort of copping out here since the change is just to an error string literal.)
Closes #15184

Differential Revision: D5499445

Pulled By: javache

fbshipit-source-id: 8051869feb5fe5fc630516972775c134f6e41a04

Author: terribleben

React/Executors/RCTJSCExecutor.mm

@@ -596,7 +596,11 @@ - (void)_executeJSCall:(NSString *)method
       }
     } else {
       if (!errorJSRef && JSC_JSValueGetType(ctx, batchedBridgeRef) == kJSTypeUndefined) {
-        error = RCTErrorWithMessage(@"Unable to execute JS call: __fbBatchedBridge is undefined");
+        error = RCTErrorWithMessage(@"Unable to execute JS call: __fbBatchedBridge is undefined. This can happen "
+                                    "if you try to execute JS and the bridge has not set up, for example if it encountered "
+                                    "an incomplete bundle or a fatal script execution error during startup. Verify that a "
+                                    "valid JS bundle is included with your app and that it loaded correctly, or try "
+                                    "reinstalling the app.");
       }
     }