Standardize gitlab-shell's messages to the user

We should have a standard way to show gitlab-shell's messages, like limit the characters of each message line and adding a blank line or three dashes before and after the message in order to be more apparent from the rest of the messages produced by Git itself.

Right now we show the user various messages, for example:

git grep -n "message = "

lib/gitlab_post_receive.rb:55:      message = "Create merge request for #{merge_request["branch_name"]}:"
lib/gitlab_post_receive.rb:57:      message = "View merge request for #{merge_request["branch_name"]}:"
lib/gitlab_reference_counter.rb:46:      message = "GitLab: An unexpected error occurred in writing to Redis: #{e}"
lib/gitlab_shell.rb:42:    message = "gitlab-shell: Access denied for git command <#{origin_cmd}> by #{log_username}."
lib/gitlab_shell.rb:48:    message = "gitlab-shell: Attempt to execute disallowed command <#{origin_cmd}> by #{log_username}."

git grep -n "stderr.puts"

lib/gitlab_access.rb:30:    $stderr.puts "GitLab: Failed to authorize your Git request: internal API unreachable"
lib/gitlab_access.rb:33:    $stderr.puts "GitLab: #{ex.message}"
lib/gitlab_logger.rb:8:  $stderr.puts "WARNING: Unrecognized log level #{log_level.inspect}."
lib/gitlab_logger.rb:9:  $stderr.puts "WARNING: Falling back to INFO."
lib/gitlab_post_receive.rb:116:      $stderr.puts "GitLab: An unexpected error occurred in writing to Redis: #{e}"
lib/gitlab_reference_counter.rb:47:      $stderr.puts message
lib/gitlab_shell.rb:39:    $stderr.puts "GitLab: Failed to authorize your Git request: internal API unreachable"
lib/gitlab_shell.rb:45:    $stderr.puts "GitLab: #{ex.message}"
lib/gitlab_shell.rb:51:    $stderr.puts "GitLab: Disallowed command"
lib/gitlab_shell.rb:54:    $stderr.puts "GitLab: Invalid repository path"

cc @rspeicher @DouweM @regisF