Nicer failure message when trying to install an invalid version. Fixes #285.

nvm.sh

@@ -308,6 +308,11 @@ nvm() {
 
       [ -d "$NVM_DIR/$VERSION" ] && echo "$VERSION is already installed." && return
 
+      if [ "$VERSION" = "N/A" ]; then
+        echo "Version '$VERSION' not found - try \`nvm ls-remote\` to browse available versions."
+        return 3
+      fi
+
       # skip binary install if no binary option specified.
       if [ $nobinary -ne 1 ]; then
         # shortcut - try the binary if possible.