improve uri fetcher error handling

This includes some error classes, a default
no-redirections-limit constant (which uses the value 1)
and a specific TooManyRedirects error class, which also
contains the previous response which we received
before it all went to hell.
use uri fetcher in ontohub net

Also produce more useful error message inside
of the log (server-side).
provide useful error message to git-push user

When ssh-access querying for permissions fails
for HTTP related reasons the git-push user should be
informed of the situation instead of being attacked
by a ruby stacktrace.
add safety-net error messages for ssh/git.
use STDERR consistently across git shell.
Merge branch 'staging' into 787-improve_ssh_access_error_handling

Conflicts:
	git/lib/git_shell.rb
Merge pull request #873 from ontohub/787-improve_ssh_access_error_handling

787 improve ssh access error handling