make traversing to non-existent bug IDs return a 404 instead of
 raising a system error
Patches applied:

 * brad.bollenbach@canonical.com/launchpad--bug-and-task-page-humanization--0--patch-1
   checkpoint commit

 * brad.bollenbach@canonical.com/launchpad--bug-and-task-page-humanization--1--base-0
   tag of rocketfuel@canonical.com/launchpad--devel--0--patch-1879

 * brad.bollenbach@canonical.com/launchpad--bug-and-task-page-humanization--1--patch-1
   make visiting an non-existent bug return a 404, rather than raise a system error

 * brad.bollenbach@canonical.com/launchpad--bug-and-task-page-humanization--1--patch-2
   small test fix

 * brad.bollenbach@canonical.com/launchpad--bug-and-task-page-humanization--1--patch-3
   also return the 404 for non-numeric bug ids

 * brad.bollenbach@canonical.com/launchpad--bug-and-task-page-humanization--1--patch-4
   merge from rf