lib: Add error prefixing for sysroot load and repo open

lib: Add error prefixing for sysroot load and repo open

Noticed this while writing tests for a core `ostree_sysroot_load()`
entrypoint.  And decided to do the same for `ostree_repo_open()`,
and while there also noted we had a duplicate error prefixing
for the open (more recently `glnx_opendirat()` automatically
prefixes with the path).