Non strict mode

[Julow]

Allow failures when resolving paths and references. (https://github.com/jonludlam/odoc/issues/14)

The Lookup_failures module uses a global ref to store failures. Failures are caught at the entry points of Odoc_xref2 and turned into a warning in Odoc_odoc.

This is not enough to close the issue:

• Strict mode cannot be turned on yet I thought warnings could be a good mechanism for that. They can be made fatal on master. Warnings are not enough if we need to abort the process at the first error (eg. to have the backtrace).

• There is still some exceptions raised from Lang_of and Tools They need to be declared and caught properly, I'll do that in an other PR

[Julow]

I reimplemented this on top of recent PRs, I'll open a new PR.