Improve start-up scripts to avoid common errors

One of the common issues we have is users not configuration things correctly in the accumulo-env.sh script. Rather than default to setting dirs to non-existant system variables, we should have a slightly more descriptive "/path/to/ITEM". To work nicely with this, we should check these directories if they exist, and if not error out with a descriptive message.

Lastly, we should export all system variables Accumulo will need to read (HADOOP_HOME, ZOOKEEPER_HOME, etc.) just in case a user sets the variable but doesn't export it.