don't print internal token in error message

Attempting to execute something like `exec "$test"` results in a fish internal
token (a Unicode private use char) being printed in the resulting error
message. That's obviously not desirable as well as confusing.

Fixes #3187

tests/vars_as_commands.err

@@ -0,0 +1,6 @@
+Variables may not be used as commands. In fish, please define a function or use 'eval $test'.
+fish: exec $test
+           ^
+Variables may not be used as commands. In fish, please define a function or use 'eval "$test"'.
+fish: exec "$test"
+           ^