MINOR: ssl: improve the errors when a crt can't be open

MINOR: ssl: improve the errors when a crt can't be open

Issue #574 reported an unclear error when trying to open a file with not
enough permission.

  [ALERT] 096/032117 (835) : parsing [/etc/haproxy/haproxy.cfg:54] : 'bind :443' : error encountered while processing 'crt'.
  [ALERT] 096/032117 (835) : Error(s) found in configuration file : /etc/haproxy/haproxy.cfg
  [ALERT] 096/032117 (835) : Fatal errors found in configuration.

Improve the error to give us more information:

  [ALERT] 097/142030 (240089) : parsing [test.cfg:22] : 'bind :443' : cannot open the file 'kikyo.pem.rsa'.
  [ALERT] 097/142030 (240089) : Error(s) found in configuration file : test.cfg
  [ALERT] 097/142030 (240089) : Fatal errors found in configuration.

This patch could be backported in 2.1.