block: detail internal error

Print error messages for any failure between the fork and exec. On
internal error, return a specific error code in order to distinguish
user script error and i3blocks internal error.
Revert "introduce the BLOCK_LIBEXEC env variable"

This reverts commit 2833ae631a5298bff3a71e28b91cdafc1063413c.

This new option added too much complexity, for a not so useful feature.
Moreover, it is totally feasible to keep a $SCRIPT_DIR variable in the
i3blocks config, and call i3blocks like so:

    bar {
      status_command SCRIPTS_DIR=~/src/i3blocks/scripts i3blocks
    }