Add more information to warnings such as `brokenLink`

[kallentu]

Add more information to warnings such as brokenLink here: https://github.com/dart-lang/dartdoc/blob/7ac2a1cf972ee48f0a21a49e4da56c25db4300f8/lib/src/warnings.dart#L212

Currently, the warning only shows the file where the broken link is. Would be nice to be able to pinpoint what is broken.

build-sdk-docs: Validating links...
build-sdk-docs:   error: dartdoc generated a broken link to: dart-js_interop/JSObject/JSObject.html
build-sdk-docs:   error: dartdoc generated a broken link to: dart-js_interop/JSObject/JSObject.fromInteropObject.html
build-sdk-docs:   error: dartdoc generated a broken link to: dart-js_interop/JSArray/JSArray.html
build-sdk-docs:   error: dartdoc generated a broken link to: dart-js_interop/JSArray/JSArray.withLength.html
build-sdk-docs:   error: dartdoc generated a broken link to: dart-js_interop/JSPromise/JSPromise.html
build-sdk-docs: Found 0 warnings and 5 errors.
build-sdk-docs: Documented 21 public libraries in 57.7 seconds
build-sdk-docs:
build-sdk-docs: dartdoc 8.0.2 (/home/runner/work/dartdoc/dartdoc/bin/dartdoc.dart) failed: encountered 5 errors
build-sdk-docs: #0      Dartdoc.generateDocs (package:dartdoc/src/dartdoc.dart:241:9)
build-sdk-docs: <asynchronous suspension>
build-sdk-docs: #1      Dartdoc.executeGuarded.<anonymous closure> (package:dartdoc/src/dartdoc.dart:268:9)
build-sdk-docs: <asynchronous suspension>