share kakrc: Use fail in the :colorscheme command

When an invalid colorscheme is selected from the user configuration
file, the editor doesn't report any errors at startup unless the
`fail` command is used.

share/kak/kakrc

@@ -21,7 +21,7 @@ def -params 1 -docstring "colorscheme <name>: enable named colorscheme" \
     if [ -n "${filename}" ]; then
         printf 'source %%{%s}' "${filename}"
     else
-        echo "echo -markup '{Error}No such colorscheme'"
+        echo "fail 'No such colorscheme'"
     fi
 }}