Lexing error in an opam file prevents further updates

[let-def]

1. Make a lexing error in a ".opam" file in a custom repository, e.g. depends: ["a","b"] instead of depends:["a" "b"].
2. Update the repository with opam update <local>. Opam will first copy all data, then complain about the lexing error when processing the local copy.
3. It's no longer possible to fix the issue in the user repository: when invoked, opam will first check its local cache and terminate with the same complain.