Fix missing pattern handling

[geoffp]

Summary of changes:

• if a requested pattern partial is missing, say so; if there's compilation error, print the output from the exception.
• if you've tried to call a non-existent pattern with _.renderNamedPartial('atom-not-there', obj), say so in the error message
• beautiful and helpful error reporting during underscore template rendering
• Shift template compilation time to the partial registration phase