If HTTP requests fails without connection suggest --offline

[jonasfj]

TODO:

• [ ] Find out how requests fail if there is no network access.
• [ ] Suggest in error message that the users tries with --offline, if they don't have an internet access at the moment.

Careful that we don't recommend --offline whenever someone is blocked by proxy or other thing.

Goal: If people are trying to do pub get without internet access, how are they supposed to discover --offline if the error message doesn't inform them? They can't google for an answer..

[sigurdm]

On macos I get:

 HTTP error:
    SocketException: Failed host lookup: 'pub.dartlang.org' (OS Error: nodename nor servname provided, or not known, errno = 8)

[tusharojha]

On Windows using git bash I get:

Resolving dependencies...
It looks like pub.dartlang.org is having some trouble.
Pub will wait for a while before trying to connect again.
Git error. Command: `git clone --mirror ...

[sigurdm]

Still relevant