Make sure files are readable when traversing source files.

[vic]

Mill was trying to read all files found under the source directory to
create a digest for each of them. This was causing an error for broken
symlinks.

At first I believed temporary files should be ignored to avoid this problem,
and asked at the gitter channel how to go about this, but overriding the sources
task as suggested
by @lihaoyi didn't actually help, because it was T.sources macro who was actually
triggering the exception.

So, on a simple scala project, editing a file with Emacs, creates a link file, like:

vic@oeiuwq ~/h/foo> ls -la foo/src/
total 8
drwxr-xr-x  4 vic  staff  128 Sep  1 12:23 .
lrwxr-xr-x  1 vic  staff   22 Sep  1 12:23 .#hello.scala -> vic@oeiuwq.local.10748
drwxr-xr-x  3 vic  staff   96 Sep  1 12:22 ..
-rw-r--r--  1 vic  staff   12 Sep  1 12:22 hello.scala

This patch only makes sures that the files (or the symlink here) is actually
readable before trying to digest it.

Fixes #402

[lihaoyi]

Perhaps using a try-catch around the else block would be a better solution? If any IOException appears here, just do nothing

[vic]

@lihaoyi Hm, not sure if using a try-catch is the way we should go. I mean, since we are trying to create an InputStream for the file and read from it, we should at least check if the file is readable (which is what this PR does), but if then we try to read it and somehow there's an error reading it (say it's on a network filesystem and network goes down), I believe we should let the IOException go and not just ignore it.

Do you have a strong opinion about using the try-catch ?

[lihaoyi]

No i don't have strong opinions, this looks good