Improve build script

h2/build.bat

 @echo off
 if "%JAVA_HOME%"=="" echo Error: JAVA_HOME is not defined.
-if "%1"=="clean" rmdir /s /q temp | rmdir /s /q bin
+if "%1"=="clean" rmdir /s /q temp bin 2>nul
 if not exist temp mkdir temp
 if not exist bin mkdir bin
 "%JAVA_HOME%/bin/javac" -sourcepath src/tools -d bin src/tools/org/h2/build/*.java