Update error message when Java or other required systems components aren't available on the user's system.

[jmelott]

Add the following link to the error message if any system requirement is not met:
https://md-anderson-bioinformatics.github.io/NGCHM-R/articles/Installation.html

For instance, add message to user pointing them to the link to the system requirements if they try to export a heatmap but Java isn't available on the user's system.
(See issue #1 for example when this would have been useful to the user.)

[marohrdanz]

The goal of this issue might have been addressed in #46 (commit d65451a).