fs[Abbr `a`] where a is not in assumptions gives bad error message

[ordinarymath]

Given a proof like

Theorem foo:
  FOO a b
Proof
  fs[Abbr`NON_EXISTENT`]

Gives the error message `Exception raised at Tactical.FIRST_ASSUM:

Exception- HOL_ERR {message = "", origin_function = "FIRST_ASSUM", origin_structure = "Tactical", source_location = ?>} raised ` When the error message should be about how NON_EXISTENT does not exist.