Do not exit on build error

[notramo]

plenti serve currently exits when it encounters a build error.
This is quite unconventional, as other tools behave differently in this case.

When a build error happens, instead of exiting, Plenti should render a HTML page of the error, and continue watching for file changes. When a file is changed, automatically rebuild, then serve the new version (or, in case of another error, serve the new error as HTML). This is what most web frameworks/SSGs are doing.

[jimafisk]

Great idea @notramo, we could implement something like this when using the local server (plenti serve). Since the deployed applications rely heavily on CI to run, I do want to continue to throw errors that halt the build to avoid shipping a broken site.