Caught Exceptions should be logged

[lukpueh]

The _build_installer() function in html/views.py catches exceptions that result from xmlrpc communication with custom installer builder and does not give verbose feedback to the user or developer.
Caught exceptions should be repred to the log.

[lukpueh]

XMLRP Calls to the Custom Installer Builder are especially hard to debug. E.g. a wrong but existing SEATTLECLEARINGHOUSE_INSTALLER_BUILDER_XMLRPC URL raises something like ExpatError: junk after document element: line 1, column 47.