better error message for rbenv prefix system

Have `rbenv prefix` handle the case where system Ruby is not installed,
i.e. `rbenv which ruby` doesn't find ruby in PATH.

Fixes #362

test/prefix.bats

@@ -23,3 +23,17 @@ load test_helper
   RBENV_VERSION="system" run rbenv-prefix
   assert_success "$RBENV_TEST_DIR"
 }
+
+@test "prefix for invalid system" {
+  USRBIN_ALT="${RBENV_TEST_DIR}/usr-bin-alt"
+  mkdir -p "$USRBIN_ALT"
+  for util in head readlink greadlink; do
+    if [ -x "/usr/bin/$util" ]; then
+      ln -s "/usr/bin/$util" "${USRBIN_ALT}/$util"
+    fi
+  done
+  PATH_WITHOUT_RUBY="${PATH/\/usr\/bin:/$USRBIN_ALT:}"
+
+  PATH="$PATH_WITHOUT_RUBY" run rbenv-prefix system
+  assert_failure "rbenv: system version not found in PATH"
+}