i3bar: Print error message when status_command fails

Add a function to i3bar to print an error message in the status line
when the child process invoked by status_command fails to provide
input that can be displayed as a statusline.

When the child provides JSON that cannot be parsed, alert the user and
convey a short message provided by yajl communicating the specific
problem.

When the child (or the shell executing the status command) exits
unexpectedly, alert the user and display the exit code. The cases where
the status command is not executable or not found in the user's PATH are
treated specially.

fixes #1130