Firstly this highlights the importance of taking regular backups particularly before making any major changes including adding extensions or hacking code.
Second it would have been possible to work around the problem by editing the database to deselect the JA Purity template as default and choose one of the other templates in the default installation.
Third, the error message you are getting would appear to be indicating a missing/corrupted part of the Joomla! package.
Finally there is quite a bit about permissions in the Joomla! Docs wiki but not so much about ownership, but these are actually not Joomla! issues but they are - in your case - Ubuntu, and Apache, MySQL, and PHP issues so perhaps researching these areas may be of benefit rather assuming Joomla! docs will cover everything.
Thanks for the reply. As I stated in my original post, other templates worked fine. That was not the issue. I could change the default template at will. I did not want to go into production with something broken, not knowing how it was broken and therefore unable to take any kind of measures if something similar happened after going into production. As far as I could tell, all I had done was change the Logo Text field and hit 'save.' Once saved, the damage appeared to be irreparable.
I can certainly rm -Rf
the entire installation a second time and unpack it again. I don't understand what, exactly, could have become "missing/corrupt" as a result of tar xzf <Joomla package>
As far as documentation goes, I am in real life a consultant specializing in a software package every bit as unwieldy and complicated as Joomla. In my head, and in my available documentation, I have a wealth of information about the impact that various other softwares have on our application. I have spent untold hours fixing "issues" that were caused by misconfiguration of these other applications and not by our tool itself. Therefore, I do not regard it as in the least unreasonable to expect that Joomla would have similar information in its documentation. YMMV.