add info in error message for LSF interpolations

linetools/spectra/lsf.py

@@ -469,7 +469,7 @@ def interpolate_to_wv0(self, wv0):
                     warnings.warn(
                         "LSF may result from extrapolation outside wavelength range characterized for current grating.")
             else:
-                raise ValueError("wv0 too far outside range of defined LSFs. Perhaps you've chosen the wrong grating?")
+                raise ValueError("wv0={:.2f}A too far outside range of defined LSFs. Perhaps you've chosen the wrong grating?".format(wv0))
         lsf_vals = np.array(lsf_vals)
 
         # normalize