In the scenario where either the ssl certificate or key files can't be read
by the user running Logstash, e.g. due to permissions, an error is generated
by io.netty.handler.ssl.SslContextBuilder.forServer communicating that these
files don't contain valid content:

"java.lang.IllegalArgumentException: File does not contain valid certificates"
or
"java.lang.IllegalArgumentException: File does not contain valid private key"

This is often misleading, causing users to question if their certificates are
created incorrectly. So this commit checks first if these files can be read,
and throws an exception dedicated to this error.

bump to 6.0.10

closes #197