Make sure there is data for SOMD2 too.

python/BioSimSpace/FreeEnergy/_relative.py

@@ -1435,6 +1435,7 @@ def _analyse_amber(work_dir=None, estimator="MBAR", method="alchemlyb", **kwargs
             for part in file.parts:
                 if "lambda" in part:
                     lambdas.append(float(part.split("_")[-1]))
+
         if len(lambdas) == 0:
             raise ValueError(
                 "No lambda windows were detected from the output directory names! "
@@ -1903,6 +1904,13 @@ def _analyse_somd2(work_dir=None, estimator="MBAR", method="alchemlyb", **kwargs
         temperatures = [x for _, x in sorted(zip(lambdas, temperatures))]
         lambdas = sorted(lambdas)
 
+        if len(lambdas) == 0:
+            raise ValueError(
+                "No lambda windows were detected from the output directory names! "
+                "Ensure that the directory being analysed contains sub-directories"
+                "name e.g `lambda_0`, `lambda_1` containing the output files."
+            )
+
         # Check that the temperatures at the end states match.
         if temperatures[0] != temperatures[-1]:
             raise ValueError("The temperatures at the endstates don't match!")