Should give people feedback when the links don't work

[eldang]

Currently if you try using this with a non-existent HB number or a wrongly formatted biennium, it will still try to open all the tabs. It's probably not that hard to (a) use AJAX calls to check if a page exists before trying to open a tab with it and (b) put an error message up if the status page for the requested bill doesn't exist (clearest indicator that the number is wrong).

NB: this interacts with #2 & #4