RecommendedCutoffMixin: improve error messages (#86)

Improve the error message in two cases:

* When deleting the final stringency with the `delete_cutoffs` method.
* An element requested with the `get_recommended_cutoffs` method does
  not have a pseudo in the pseudo family.

aiida_pseudo/groups/mixins/cutoffs.py

@@ -220,21 +220,25 @@ def delete_cutoffs(self, stringency) -> None:
         try:
             if stringency == self.get_default_stringency():
                 self.delete_extra('_default_stringency')
-                warning += f'`{stringency}` was the default stringency of this family. '
+                warning += f'`{stringency}` was the default stringency of this family.'
                 assign_new_default = True
             else:
                 assign_new_default = False
         except ValueError:
             assign_new_default = True
 
         if assign_new_default:
-            if len(cutoffs_dict) == 1:
+            if len(cutoffs_dict) == 0:
+                warning += (
+                    ' Since no other stringencies are defined for this family, no new default can be specified.'
+                )
+            elif len(cutoffs_dict) == 1:
                 final_stringency = list(cutoffs_dict.keys())[0]
                 self.set_default_stringency(final_stringency)
-                warning += f'Setting `{final_stringency}` as the default since it is now the only defined stringency.'
+                warning += f' Setting `{final_stringency}` as the default since it is now the only defined stringency.'
             else:
                 warning += (
-                    f'Please set one of {self.get_cutoff_stringencies()} as the new default stringency with the '
+                    f' Please set one of {self.get_cutoff_stringencies()} as the new default stringency with the '
                     '`set_default_stringency` method.'
                 )
 
@@ -276,6 +280,7 @@ def get_recommended_cutoffs(self, *, elements=None, structure=None, stringency=N
         :return: tuple of recommended wavefunction and density cutoff.
         :raises ValueError: if the requested stringency is not defined for this family.
         :raises ValueError: if optional unit specified is invalid.
+        :raises ValueError: if the family does not have a pseudo for one of the elements (of the structure).
         """
         if (elements is None and structure is None) or (elements is not None and structure is not None):
             raise ValueError('at least one and only one of `elements` or `structure` should be defined')
@@ -299,6 +304,9 @@ def get_recommended_cutoffs(self, *, elements=None, structure=None, stringency=N
 
         for element in symbols:
 
+            if element not in cutoffs:
+                raise ValueError(f'family does not contain a pseudo for element `{element}`.')
+
             if unit is not None:
                 current_unit = self.get_cutoffs_unit(stringency)
                 values = {k: U.Quantity(v, current_unit).to(unit).to_tuple()[0] for k, v in cutoffs[element].items()}