Fix display of internal errors
Cut long line
Fix race condition when reading the output of `ocamlc -version`

Fix #557

To avoid the race, we add the PID in the log name and we ensure that the file does not exist when we open it, as suggested by @edwintorok.
More long lines
Distinguish between `ocamlc is not present` and `ocamlc fails to run` when checking if a system compiler is present. This enables us to display more precise error messages to the user.

(This was useful to check if the previous commit really fixed #557, as the repro steps are to run a 'fork' bomb -- in this case you want OPAM to report that correctly to user)