Add check for build targets in create script

I installed the android sdk via homebrew, which does not install any build targets by default.

That resulted in the create throwing the following somewhat cryptic error.

An unexpected error occurred: "$ANDROID_BIN" create project --target $TARGET --path "$PROJECT_PATH" --package $PACKAGE --activity $ACTIVITY >&/dev/null exited with 1

This just adds a check that the variables that are set to the values of the build targets are properly set, and exit if they are not.