configtxgen: Marshal called with nil - when certificate in msp/cacerts is missing

### Description

When "MSPDir"-parameter in configtx.yaml is definied and all certificates available but the ca-cert.pem in the cacerts subdirectory, then an unspecified error occurs: proto: Marshal called with nil

Of course, this is a missconfiguration of the CA, but the inexistance of a file can be better reported to find the error.

### Steps to reproduce

Move the ca-cert.pem from msp/cacerts directory to somewhere else.