pull: Update key loading function to match error style

pull: Update key loading function to match error style

This code wasn't written with idiomatic GError usage; it's not standard
to construct an error up front and continually append to its
message.  The exit from a function is usually `return TRUE`,
with error conditions before that.

Updating it to match style reveals what I think is a bug;
we were silently ignoring failure to parse key files.