Add a text based mill lock.

[He-Pin]

Motivation:
When working on com-lihaoyi/cask#159 , I see quit ... is been used by another mill process and the mill lock file gives me nothing

I would suggest making the mill lock text-based which contains more information

[lihaoyi]

It should already tell you what's running, maybe cask is on an old version?

lihaoyi mill$ ./mill version
Another Mill process is running '__.compile', waiting for it to be done...