iOS: add null check on create impeller context

[cbracken]

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/flutter#157489
Issue: b/378790930

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

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[hellohuanlin]

good catch!

[cbracken]

This is mostly just passing the buck, but consistent with the metal device lookup below. There are tests that assume this initialiser can return nil, and one place in the code where we're attempting to set a boolean based off an assumption (well, a transitive assumption) that this initializer may return nil:

engine/shell/platform/darwin/ios/framework/Source/overlay_layer_pool.mm

Line 63 in 8d3c718

bool impeller_enabled = !!ios_context->GetImpellerContext();

Another alternative would be to just FML_CHECK that it's not null and crash with a decent log message if we fail to create an impeller context. I mean... good things aren't going to happen in this scenario either way.

[jonahwilliams]

LGTM

[stuartmorgan-g]

test-exempt: changes logging of a fatal error, which we don't have a mechanism to test