tell: check for empty token list

This avoid generating twice "Done" outline
Chunk: use sum rather than option to return failure

Sometimes, Chunk_parser fails to extract information from outline.
This can be because we reached end-of-buffer, or because the phrase was
malformed and parser recovery failed.

Though mainly useful for debugging, it was not possible to distinguish
between these two cases.
Maximizing scope still needed, fix later
Adjust env_at / Browse.near

Conflicts:
	browse.ml
	command.ml