Improve logging for fetch tasks

[chaws]

This issue targets improving logging on fetch tasks. Milosz is suggesting that we replace the uuid with backend + job id.

[mwasilew]

I would also touch error reporting with email. Sometimes it's really hard to tell where the problem happened. What is usually missing:

• instance when the problem happened (it sometimes can be deduced from paths)
• called method name
• variable values used in the call

In theory django error reporting provides all of that but in practice it's often not very useful.

[chaws]

This solves half of the problem: https://github.com/Linaro/squad/pull/839