Make `sbt clean` error free

[metasim]

very minor...

sbt clean forwards errors from CMake subsystem. Would be nice to know that the native build artifacts got deleted propertly.

❯ sbt clean
...
[info] set current project to pdal-jni (in build file:/Users/sfitch/Coding/OSS/PDAL-Java/)
[error] CMakeFiles/pdaljni.2.6.dir/build.make:64: CMakeFiles/pdaljni.2.6.dir/compiler_depend.make: No such file or directory
[error] gmake[2]: *** No rule to make target 'CMakeFiles/pdaljni.2.6.dir/compiler_depend.make'.  Stop.
[error] gmake[1]: *** [CMakeFiles/Makefile2:100: CMakeFiles/pdaljni.2.6.dir/clean] Error 2
[error] gmake: *** [Makefile:142: clean] Error 2

[pomadchin]

That's unfortunately how CMake delivers clean message (https://github.com/sbt/sbt-jni/blob/979c1477087cf76ac33e9c07062a32a7bb063440/plugin/src/main/scala/com/github/sbt/jni/build/CMake.scala#L51-L56)

Wondering if its better to redirect it into the regular out / warning output.