There are at least two ways that I know of to get the error. The first is using a global variable: $! which is always set to the last error that occurred. The second is by explicitly capturing the error when you rescue:
begin
# do something that fails...
rescue => error
# error and $! are equivalent here
end
Either one will let you inspect or print out the backtrace using either:
$!.backtrace # => array of backtrace steps
error.backtrace # => same error