[CMake] cmake should try harder to abort on invalid input

[Dan Kegel]

I was mystified by the following problem:

add_executable(x x.c)
add_custom_command(
   TARGET x
   POST_BUILD
   command sh ${CMAKE_SOURCE_DIR}/frob.sh x
)

Making this project didn't execute frob.sh.

Turns out cmake 2.8.9 silently ignores the command because... it's lowercase.
Writing COMMAND instead of command makes things work.

Can cmake please throw an error in that case?