tools: Correct insert keys behavior on pem files

Insert keys would erroneously process pem files
with openssl headers in them. Also, the tool would
be fooled into attempting to use pem files that
had private keys and other things in the format.
This patch strengthens the formatting requirements
and increases the verboseness of error messages
when processing pem files.

Change-Id: I03353faaa641233a000d1a18943024ae47c63e0f