Changed errors_SString to errors_SS.

Mainly to reduce possibly confusion from the original name with SString. I have found myself sometimes thinking that a function returned SString because the SString in errors_SString lined up well over an argument that had that type, so I would like to prevent that.
Removed a comment about the second enum and made it into its name.

It didn't seem necessary as its own comment when it could be just the name of the enum.
Fixed to_X_sstring() not catching errors from new_sstring().
Added missing checks for when memory doesn't get allocated.
Clarified what remove_sstring() does and doesn't do.
Added missing checks for errors reported by remove_sstring() in trim_sstring().
Fixed functions checking wrong things.