It is impossible to prevent the SQLCMDreturn of output messages without errors by passing a parameter to it.
However, you can redirect error messages to STDERR, and then forward all other messages to NUL.
This is done by passing a parameter -r. From a book online :
-r [0 | 1] msgs to stderr
(stderr). 0, 11 . 1, , PRINT, . , -o. .
-r , , , :
sqlcmd -Q "select 1 as a; select 1/0 as b" -E -r1 1> NUL