lib/util.sh: added to_file (), which auto-removes a command's output file on error (like make's .DELETE_ON_ERROR)

lib/util.sh: echo_run_extern, extern: fixed/added comments to indicate that echo_run_extern echoes and controls stderr of an external command, while the extern alias does this for all external commands

lib/util.sh: auto-echo common external commands, such as rm

lib/util.sh: echo_run_extern, extern aliases: alias-expand next word (the command) by adding trailing space to alias def

bugfix: lib/util.sh: exceptions: log_e: must include `declare e` in the alias and not when save_e is called, so that $e is a local var of the caller. this bug did not appear in testing because the save_e alias, which re-scopes $e within log_e (), was not expanded inside log_e () (since new aliases were not loaded between save_e and log_e ()).

lib/util.sh: exceptions: added log_e (), which prints a "command exited with error" message (like make) when applicable

lib/util.sh: exceptions: try_ (): renamed to try () and use the function keyword to distinguish it from the alias

lib/util.sh: exceptions: try_ (): use new save_e

lib/util.sh: exceptions: added save_e

lib/util.sh: exceptions: end_try* aliases: use new rethrow*