Corrected regressions in zsh context and paging.

is_context_cli_function() now successfully matches both autoloaded and
non-autoloaded functions. Likewise, page_stdin() now pages standard
input without erroneously duplicating such input to standard output.
Variable type handlers (e.g., die_unless_variable_string()) now throw
exceptions with better formatted messages.

"And we did slake our thirst
 Unquellingly,
 Unquenchingly,
 Unflinchingly.

 Urchins in the
 umber dark did
 ravish my stark
 indignity of a
 slavish myopia.

 My sunlit breath
 Upended into deathly
 Placenta's
 Placidity –
 Tranquillity –
 Oneiric docility."