Fix invalid DIR parameter passed to `read-file-name'

The filename of the current buffer (instead of the dirname) was passed
as DIR parameter), resulting in an invalid directory where emacs starts
completion.

Also ensure that the project file exists.
Merge pull request #441 from juergenhoetzel/master

Fix invalid DIR parameter passed to `read-file-name'
Update FSC to 0.0.43.

There are a lot of bug fixes in FSC since 0.0.36. This commit doesn't begin to take advantage of the new async methods in FSC, but puts us in a better place to begin improving responsiveness.
Merge pull request #443 from rojepp/fsc0.0.43

Update FSC to 0.0.43.
Unbreak FSharp.AutoComplete
Merge pull request #445 from rojepp/fsc0.0.43

Unbreak FSharp.AutoComplete