Adds error handling to rbenv-sh-shell

libexec/rbenv-sh-shell

@@ -1,2 +1,16 @@
 #!/bin/sh
-echo "export RBENV_VERSION=$1"
+
+if [ -z "$1" ]; then
+  echo "Please specify one of the following Ruby versions to use:" >&2
+  echo "" >&2
+  echo "$(rbenv-versions)" >&2
+  exit 1
+fi
+
+version=$1
+
+if [ -d "$HOME/.rbenv/versions/$version" ]; then
+  echo "export RBENV_VERSION=$version"
+else
+  echo "rbenv: version \`$version' is not installed" >&2
+fi