iOS: add null check on create impeller context (flutter/engine#56952)

In the case where `CreateImpellerContext` encounters an error while creating an `impeller::ContextMTL`, we logged an error, returned nullptr, then immediately dereferenced the null pointer. Now, rather than crash due to a segfault, we now intentionally abort with an appropriate error message.

This adds checks in the `FlutterDarwinContextMetalImpeller` initialiser that aborts with an appropriate error message if impeller context creation fails, Metal device creation fails, or texture cache creation fails.

Rather than bailing out and returning nil from the initialiser to pass the buck to the caller, we terminate since without a graphics context, the app won't be able to render anything to begin with.

Issue: flutter#157489
Issue: [b/378790930](http://b/378790930)

No test changes since this just changes an accidental crash to an intentional crash.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

engine/src/flutter/shell/platform/darwin/graphics/FlutterDarwinContextMetalImpeller.mm

@@ -25,14 +25,8 @@
       std::make_shared<fml::NonOwnedMapping>(impeller_framebuffer_blend_shaders_data,
                                              impeller_framebuffer_blend_shaders_length),
   };
-  auto context = impeller::ContextMTL::Create(shader_mappings, is_gpu_disabled_sync_switch,
-                                              "Impeller Library");
-  if (!context) {
-    FML_LOG(ERROR) << "Could not create Metal Impeller Context.";
-    return nullptr;
-  }
-
-  return context;
+  return impeller::ContextMTL::Create(shader_mappings, is_gpu_disabled_sync_switch,
+                                      "Impeller Library");
 }
 
 @implementation FlutterDarwinContextMetalImpeller
@@ -41,11 +35,9 @@ - (instancetype)init:(const std::shared_ptr<const fml::SyncSwitch>&)is_gpu_disab
   self = [super init];
   if (self != nil) {
     _context = CreateImpellerContext(is_gpu_disabled_sync_switch);
+    FML_CHECK(_context) << "Could not create Metal Impeller Context.";
     id<MTLDevice> device = _context->GetMTLDevice();
-    if (!device) {
-      FML_DLOG(ERROR) << "Could not acquire Metal device.";
-      return nil;
-    }
+    FML_CHECK(device) << "Could not acquire Metal device.";
 
     CVMetalTextureCacheRef textureCache;
     CVReturn cvReturn = CVMetalTextureCacheCreate(kCFAllocatorDefault,  // allocator
@@ -55,10 +47,7 @@ - (instancetype)init:(const std::shared_ptr<const fml::SyncSwitch>&)is_gpu_disab
                                                   &textureCache  // [out] cache
     );
 
-    if (cvReturn != kCVReturnSuccess) {
-      FML_DLOG(ERROR) << "Could not create Metal texture cache.";
-      return nil;
-    }
+    FML_CHECK(cvReturn == kCVReturnSuccess) << "Could not acquire Metal device.";
     _textureCache.Reset(textureCache);
   }
   return self;