RFR: JDK-8176467 --with-cacerts-file should fail during configure if file does not exist

[Magnus Ihse Bursie]

Running bash configure --with-cacerts-file=non-existing-file currently 
gets past configure but fails during build. Configure should tighten the 
check.

The intention is to push this once the consolidated repo opens.
Bug: https://bugs.openjdk.java.net/browse/JDK-8176467
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8176467-check-with-cacerts-file-argument/webrev.01

/Magnus