Add SHELL_ERROR and SHELL_EXIT_CODE magic variables to psql.

Add SHELL_ERROR and SHELL_EXIT_CODE magic variables to psql.

These are set after a \! command or a backtick substitution.
SHELL_ERROR is just "true" for error (nonzero exit status) or "false"
for success, while SHELL_EXIT_CODE records the actual exit status
following standard shell/system(3) conventions.

Corey Huinker, reviewed by Maxim Orlov and myself

Discussion: https://postgr.es/m/CADkLM=cWao2x2f+UDw15W1JkVFr_bsxfstw=NGea7r9m4j-7rQ@mail.gmail.com