Consider exposing CustomError

[fvictorio]

As part of answering this issue, I created an example that makes the compile task fail if there are solc warnings.

The problem is this:

https://github.com/fvictorio/hardhat-examples/blob/2eb356307f92b58a5b6cd5ed9700bdb4ed4d3642/fail-on-warnings/hardhat.config.js#L14

This produces an ugly:

An unexpected error occurred:

Error: Compilation produced 1 warnings

which always includes the stack trace.

In theory you could use HardhatPluginError, but if this is a custom thing in your project, then it's not really a plugin. In this case, you need to add a dummy plugin name, which feels wrong.

We should consider exposing the CustomError class somewhere, or maybe some other class that extends from it.