Improve logging in SCEP helper
    
    Always check return value of cm_pkcs7_verify_signed() and return
    a unique error message.
    
    Change log level from 1 to 0 for all errors in scep.c and pkcs7.c
    so they appear by default.
    
    Centralize logging across scep.c and pkcs7.c to reduce code
    duplication.
    
    Check the return code to cm_pkcs7_verify_signed in all cases.
    
    Add the last available message, if any, to the error returned
    via stdout to certmonger as a hint to what is going on.