ENH: do not fallback to default font if explicitly was asked not to

ATM documentation (and no tests at all) was not conclusive on what would happen if font is not found and ```fallback_to_default``` was explicitly set to False.  So instead of just whining I decided to "patch" it introducing explicitly throwing exception if no font was found and no fallback was requested.  Before that, it also kinda didn't make sense because requesting not to fallback to default resulted actually into falling back to default ;)