clarify error message

Author: cwhanse

pvlib/clearsky.py

@@ -817,10 +817,13 @@ def detect_clearsky(measured, clearsky, times=None, infer_limits=False,
         tools._get_sample_intervals(times, window_length)
 
     if samples_per_window < 3:
-        raise ValueError(f"Window of length {window_length} found"
-                         " Each window must contain at least"
-                         " three data points; {samples_per_window} results"
-                         " from time interval of {sample_interval}")
+        raise ValueError(f"Samples per window of {samples_per_window}"
+                         f" found. Each window must contain at least 3 data"
+                         f" points."
+                         " Samples per window is calculated from window_length"
+                         " ({window_length} found) and the data interval"
+                         f" ({sample_interval} found)."
+                         " Increase window_length.")
 
     # if infer_limits, find threshold values using the sample interval
     if infer_limits:

pvlib/tests/test_clearsky.py

@@ -680,7 +680,7 @@ def test_detect_clearsky_not_enough_data(detect_clearsky_data):
 
 def test_detect_clearsky_window_too_short(detect_clearsky_data):
     expected, cs = detect_clearsky_data
-    with pytest.raises(ValueError, match="Window of length"):
+    with pytest.raises(ValueError, match="Samples per window of "):
         clearsky.detect_clearsky(expected['GHI'], cs['ghi'], window_length=2)