Add "--build-order" to cat (and a couple minor bugfixes)

[tianon]

• fix parsing of empty BASHBREW_ARCH_NAMESPACES
• better (more contextual) error messages from bashbrew remote arches

[tianon]

(This is something we've emulated in a few places with bashbrew list --build-order ... | xargs bashbrew cat but that provides a result that's different in kind of subtle ways and I don't see a good reason not to support this here 😅)

[tianon]

To be explicitly clear: this only sorts the top-level arguments to cat in build order - any deeper entry-level sorting would be accomplished in the format string.