Handle errors when couldn't download config or compose

[dpordomingo]

fix #245 motivated by https://github.com/src-d/sourced-ce/issues/241

If the user initializes or tries to download a config without access to the internet, the error is a bit cryptic.

This PR adds clearer error messages in such situations and also points to some docs explaining why is that assets needed.

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When there is no local docker-compose installation, and it could not be downloaded:

• because of network error:

• because of write error when saving the file:

When trying to download and activate a new docker-compose.yml config, but the process failed:

• because of network error:

• because of write error when saving the file:

disclaimer

This PR does not explain how to run sourced offline, which could be done separatelly if needed.

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• [ ] I have updated the CHANGELOG file according to the conventions in keepachangelog.com
• [x] This PR contains changes that do not require a mention in the CHANGELOG file

[smacker]

My 5 cents. I agree with Carlos to put instruction on how to run it without the Internet into FAQ for the same reasons he explained above. Plus people tend to skip reading the documentation if there is too much text (I'm a good example of this) so keeping only what is really important increases chances that it will be actually read.

[dpordomingo]

Thanks for your deep review. I rebased on top of the current master, and apply the changes you suggested:

• moved docs into FAQ
• avoid defer
• separate errors caused by the network from the ones caused when the config could not be written.

[dpordomingo]

Inline suggestions applied, thanks @carlosms ! @se7entyse7en your suggestions about removing defer were addressed following the conversation I had with @smacker on last Friday. Does it suit you?