[rest/stargate] Improve error response when trying to create a scanner on a nonexistant table

Review Request #977 - Created Oct. 6, 2010 and submitted

Andrew Purtell
Return 404 when an attempt is made to create a scanner on a nonexistent table. 
New unit test, passes.
  1. Looks good to me.  I think in other parts of code we'll do something like: if (e instanceof SomeExceptionType) but I guess these two ways are basically equivalent?  Or the instanceof way would cover more generic stuff like seeing if something is an IOE but would work even if the actual constructed class was something else but that extended IOE.
    1. Right, I was thinking of using Class.isAssignableFrom but forget if that reads left-to-right or right-to-left so I gave up. instanceof is better than equality, I'll use it.