Re-fix Webpack 5 watch not failing on error

[appzuka]

See: #1204

Originally, declaration file and errors were emitted during the compiler.afterCompile hook. In webpack5 it is no longer allowed to emit assets in the afterCompile hook so I moved this to the compilation.afterProcessAssets hook. But then as reported here errors were not emitted in watch mode. So I made the change here which is to emit errors during the compiler.afterCompile hook but to emit declaration files in the loader._compilation.afterProcessAssets hook.

But then it was reported that declaration files are not emitted in subsequent runs during watch mode. This was traced to loader._compilation not being persistent so we need to use a compilation hook to get the compilation so we can add the afterProcessAssets hook. With hindsight, this was also the cause of errors not being emitted in the previous step and would have been a better solution.

But now I have broken error reporting again. I can see this is because when we emit the declaration files the list of files to check for errors is reset, so when we check whether there are any errors in the afterCompile stage no files are checked.

If I change the code so that errors and declaration files are emitted at the compiler.afterProcessAssets stage everything seems to work correctly for both errors and declaration files for both normal and watch builds.