Adding positions to the OPAM parse-tree

This should finally give us proper locations when anything gets wrong in
a file. I'm confident this will save people more time than it took me to
add :)
Be more lax on OPAM files errors

eg print the error but don't block. This will let people fix the
error and update again. Closes #1125
Merge pull request #1222 from OCamlPro/file-positions

Adding file positions to the internal parse-tree