caddytls: Return errors instead of nil in client auth provisioning (#7464)

Two error returns in ClientAuthentication.provision() were
returning nil instead of the actual error, silently swallowing
failures when converting PEM files to DER and when provisioning
the CA pool. This could cause mTLS client authentication to
silently fall back to the system trust store, accepting any
client certificate signed by a public CA instead of restricting
to the configured trust anchors.