This code was already using io.Writer, but was completely ignoring write
errors.
The most worrying part is how GenMarkdownTreeCustom used an unnecessary
buffer to then dump all of its contents on a file, and instead of
returning an error on file creation/writing, it would just exit the
entire program.
Since the switch from *bytes.Buffer to io.Writer, errors can no longer
be ignored. Also makes the reuse of GenBashCompletion in
GenBashCompletionFile without a buffer treat errors properly again.
Daniel Martí