/setblock-ing an observer block with any data value between 6-15 crashes the game

Verification builds: 1.2, 1.2.1, 1.2.2
Summary: An observer can only face 6 directions. Other blocks that have a limited number of states like this either say "x Data value does not exist" when set-blocking or repeats the same states again. Instead, with observers, the game just crashes.
Impact: When converting java worlds, this can cause issues.
Repro Steps: /setblock ~ ~ ~ observer 6
Observed Results: Invisible block for a second and then crash.
Expected Results: "'6' is not a valid data value for Observer." OR Placing the same state as observer 0