dumpon: provide diag info when PEM_read_RSA_PUBKEY fails

This change modifies dumpon to print out the last error from OpenSSL
when PEM_read_RSA_PUBKEY fails. This allows end-users to diagnose why
reading in RSA pubkey files fails so they can adjust the usage to meet
the needs of the command.

MFC after: 1 week

(cherry picked from commit 52b63df9b6dfc157fb0b9f61a770b64e3663dee9)