Layer.__init__: ValueError on num.of pulses is fatal, not warning

error message changed accordingly

src/chained_requests.py

@@ -44,9 +44,9 @@ class Layer:
             except ValueError:
                 # Since number_of_pulses is required, if it can't be calculated raise error:
                 raise ValueError("""
-    Warning: either Duration or Repetition Rate are empty or invalid.
-    If you think this is an error, please edit your eLabFTW entry and retry.
-    Setting Number of Pulses to NoneType.
+    Fatal: either Duration or Repetition Rate are empty or invalid.
+    This has to be an error, since these fields are required by the NeXus standard.
+    Please edit your eLabFTW entry and retry.
                 """)
             # TO-DO: remove trailing space on eLabFTW's template for deposition layers
             self.temperature = self.extra["Heater temperature "]["value"] # TYPO: trailing space, must fix on elabftw