Fix retrying on network error in Firefox and Safari

[Woyken]

Fixes #51

Added missing dot(".") at the end of Firefox error message.

[sindresorhus]

I wonder if this applies to Safari too as it uses a dot in this issue #41, but was also added without a dot.

// @peterj35

[Woyken]

Unfortunately, I don't have safari to check myself.

This Gist has the error written in comment with a dot "TypeError: The Internet connection appears to be offline."

Also quick google search revealed few images where it looks like it probably does end with a dot

(Source)

I'll add it too