replace fail module with thiserror

[waynr]

Addresses #75
Builds on #95 (because the compiler warnings were annoying me)

While this is largely a cleanup PR it does make a couple significant changes:

• Renames ErrorKind -> DockworkerError (I have no problem with changing this back, I just thought it was better to be more descriptive for the sake of external uses that might import it)
• Removes the var field from Dockworker::InvalidUri since uri parsing is only done during creation of HyperClient connections whose errors are ultimately wrapped in Dockworker::CouldNotConnect which already includes the same information.

[waynr]

I'm not too familiar with CircleCI, but it appears that in order to view the CI job results I have to log into their website and I'd rather not.

@eldesh Is there a way to make the CI job results visible to shmucks like me?

[eldesh]

@dalance (#122) made a similar (but newer) change. Thank you very much.