Allow pra to recover when fetching raises errors

I did this so that pra will not crash when fetching pull requests
fails (e.g. due to http timeout, missing vpn connection, laptop was
closed for 5 minutes, etc). If there is an error during fetching, the
short message will be printed to the screen and the full error with
stack trace will be logged to ~/.pra.errors.log, and pra will keep
running and retry later.
Let pull sources fail without impacting the others

I did this so if a fetch from a pull source fails due to a problem
that doesn't affect the other sources, we can still display the pull
requests that can be fetched successfully. I also changed the error
display to print a generic message telling the user where to look to
find detailed errors.
Merge branch 'dont_crash_on_failed_fetch'