Make client work without internet

[samuel-w]

Currently the client fails when running a command without internet, even when the command is cached. For example, tldr df with internet disconnected returns

Error fetching from tldr: <urlopen error [Errno -3] Temporary failure in name resolution>

[MasterOdin]

While I get and like the general intention here of having tldr client work without internet, I dislike that we lose error reporting that a page was not found potentially due to internet connections instead of just serving up a generic "Page could not be found" error which is just going to confuse people if they do have internet (but are blocked via proxy or whatever).

We should probably "remember" the failed internet error and that if we go through all page candidates and none were found, then display the failed internet error, rather than fallback to the generic one.

[samuel-w]

OK, I've changed it.