[native_assets_builder] Output a success bool and don't throw

[dcharkes]

When more than one package with native assets has a build failure, throwing an error on the first build failure is unhelpful.

If we provide an interface of what we return from BuildRunner we can add a list of errors.

We should add some API that we can extend later. (Most likely we'd also want some way to display the errors in flutter_tools on builds that fail rather than rely on only streaming logs in the context of #33.)

#105 (comment)

[dcharkes]

Should the BuildResult have a errors or exceptions?

• https://api.dart.dev/stable/3.0.7/dart-core/Exception-class.html
• https://api.dart.dev/stable/3.0.7/dart-core/Error-class.html

These are not errors that a caller should expect or catch — if they occur, the program is erroneous, and terminating the program may be the safest response.

The launchers dartdev and flutter_tools need to terminate if native assets are failed to be built.

This indicates these should be errors.

However, we don't want to have these two packages just throw a stack trace, they should exit gracefully.
We don't throw these objects at all, we'll wrap them in a list and return them.

If dartdev and flutter_tools wanted to surface more tailored error messages rather than just copy-pasting a string from a message field (e.g. maybe we want to list the package name that caused the error), then we need more structured information than Errors typically have and we end up with Exceptions:

An Exception is intended to convey information to the user about a failure, so that the error can be addressed programmatically. It is intended to be caught, and it should contain useful data fields.

Maybe, we should not have a type that implements Exception or Error at all. And have a NativeAssetsBuilderError class of which the instances should not be thrown.

cc @mkustermann I believe we discussed this earlier, do you happen to have an example of code that has this kind of pattern already?

[dcharkes]

Based on the discussion in #107 (comment), this should be just outputting a boolean value for success, not a list of errors. The errors should be streamed to the logger.