lc.scm: Improve error handling and formatting of error messages.

* ui/lc.scm (lazycat-add, lazycat-rem, lazycat-exec)
  (lazycat-diff, lazycat-set, lazycat-get, lazycat-list): Improve
  error handling.  Improve formatting of error messages.
* ui/cli/pretty-format.scm (format-error): Improve error
  formatting.
  (format-output): Likewise.
  (format-error-message): New procedure.