(SERVER-259) Improve validate-memory-requirements! error message

Without this patch the fatal error message given to the user when RAM is less
than 1.1 * JVM heap size is the following following:

    Not enough RAM. Puppet Server requires at least 2181MB of RAM.

This is a problem because it leads the user to believe they don't have enough
RAM.  They may conclude their only recourse is to add more memory when another
recourse is to decrease the configured maximum JVM heap size.

This patch addresses the problem by augmenting the error message to
specifically call out the JVM heap setting as a possible recourse.  The message
further tries to help the user by directing them to the JAVA_ARGS, -Xms, -Xmx
options, and the locations of the configuration files on Debian and Enterprise
Linux, the most common platforms.
I understand your argument.  I don't think it's entirely correct, however - changing this to a `def` would only affect when the meminfo file is read from disk, not when the memory situation is actually checked/enforced (that happens below in `validate-memory-requirements!`).

In any case, I don't think this a big deal, so it's fine with me if you want to leave it as a `defn`.  Obviously the behavior will be the same either way, I was just thinking that a `def` made more sense conceptually.