Previously, if a bad extended diagnostic category was given, it would fail with the expected error as well as an unexpected assertion error:

```
$ ng build -c development
✔ Browser application bundle generation complete.

./src/main.ts - Error: Module build failed (from ./node_modules/@ngtools/webpack/src/ivy/index.js):
Error: Unexpected call to 'assertNever()' with value:
test
    at /home/douglasparker/Source/ng-new/node_modules/@ngtools/webpack/src/ivy/loader.js:77:18
    at processTicksAndRejections (internal/process/task_queues.js:95:5)

./src/polyfills.ts - Error: Module build failed (from ./node_modules/@ngtools/webpack/src/ivy/index.js):
Error: Unexpected call to 'assertNever()' with value:
test
    at /home/douglasparker/Source/ng-new/node_modules/@ngtools/webpack/src/ivy/loader.js:77:18
    at processTicksAndRejections (internal/process/task_queues.js:95:5)

Error: error NG4004: Angular compiler option "extendedDiagnostics.checks['invalidBananaInBox']" has an unknown diagnostic category: "test".

Allowed diagnostic categories are:
warning
error
suppress
```

The assertion comes from `ExtendedTemplateCheckerImpl`, which expects a well-formed configuration, yet the compiler would construct it even when errors were found. This commit skips constructing and running extended diagnostics if the configuration had errors, which should avoid triggering these assertion errors.

I'm unfortunately not able to actually test this change. The test passes even before the fix because the `ngc` binary and end-to-end tests [don't request diagnostics unless the configuration is considered valid](https://github.com/angular/angular/blob/ed21f5c75378e1ce717ee3d76d28c8c994209de1/packages/compiler-cli/src/perform_compile.ts#L292-L293). See [Slack](https://angular-team.slack.com/archives/C4WHZQMRA/p1642641305003800) for more details.

PR Close #44778