Don't throw Objective C exception when quitting MacVim

Currently when quitting MacVim, MMVimController will throw an exception
in `sendMessage:`. This is because MMAppController's handler for
quitting simply closes the connection, terminates the Vim processes and
quit; it doesn't individually shut down each Vim controller cleanly.
This is actually ok because we are quitting the app anyway, and it's not
terrible to just let the OS clean up (applicationWillTerminate also has
a 5 second executation timer set by macOS), but we do need to make sure
Vim controllers won't be trying to handle the now invalid connections.

Currently the exceptions will be caught by an exception handler, but
it's still not great, and could be confused with a bug, especially if
logging is enabled.

Add a way to set `isInitialized` to NO when shutting down, so that the
controllers will be blocked from trying to send connections.

src/MacVim/MMAppController.m

@@ -759,6 +759,15 @@ fsEventCallback(ConstFSEventStreamRef streamRef,
                                  andEventID:'MOD '];
 #endif
 
+    // We are hard shutting down the app here by terminating all Vim processes
+    // and then just quit without cleanly removing each Vim controller. We
+    // don't want the straggler controllers to still interact with the now
+    // invalid connections, so we just mark them as uninitialized.
+    for (NSUInteger i = 0, count = [vimControllers count]; i < count; ++i) {
+        MMVimController *vc = [vimControllers objectAtIndex:i];
+        [vc uninitialize];
+    }
+
     // This will invalidate all connections (since they were spawned from this
     // connection).
     [connection invalidate];

src/MacVim/MMVimController.h

@@ -57,6 +57,7 @@
 }
 
 - (id)initWithBackend:(id)backend pid:(int)processIdentifier;
+- (void)uninitialize;
 - (unsigned)vimControllerId;
 - (id)backendProxy;
 - (int)pid;

src/MacVim/MMVimController.m

@@ -248,6 +248,15 @@ static BOOL isUnsafeMessage(int msgid);
     [super dealloc];
 }
 
+/// This should only be called by MMAppController when it's doing an app quit.
+/// We just wait for all Vim processes to terminate instad of individually
+/// closing each MMVimController. We simply unset isInitialized to prevent it
+/// from handling and sending messages to now invalid Vim connections.
+- (void)uninitialize
+{
+    isInitialized = NO;
+}
+
 - (unsigned)vimControllerId
 {
     return identifier;