SF: Unknown variable name results in ASSERT, but should be SFH_ASSERT

[MichaelHuth]

Unknown variable names (e.g. typo) result in

  ################################
  !!! Assertion FAILED !!!
  Message: "Variable not found"
  Please provide the following information if you contact the MIES developers:
  ################################
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Stacktrace:
  SF_button_sweepFormula_display(...)#L1680 [MIES_SweepFormula.ipf]
SF_MarkErrorLocationInNotebook(...)#L2269 [MIES_SweepFormula.ipf]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Time: 2025-11-21T18:54:11+01:00
  Locked device: [- none -]
  Current sweep: [- none -]
  DAQ: [- none -]
  Testpulse: [- none -]
  Acquisition state: [- none -]
  Experiment: avgMethodTesting2 (Packed)
  Igor Pro version: 9.06B01 (56565)
  MIES version:
  
  ################################

Convert this to an SFH_ASSERT and check that location is shown properly.