Installable::getCursors: Cleanup

- Clarify doc comments, Installables::getCursors returns non-empty
  vector
- Use vector::at in Installable::getCursor instead of checking for empty
  vector and throwing an exception with error message.

src/libcmd/installables.hh

@@ -103,9 +103,13 @@ struct Installable
         return {};
     }
 
+    /* Get a cursor to each value this Installable could refer to. However
+       if none exists, throw exception instead of returning empty vector. */
     virtual std::vector<ref<eval_cache::AttrCursor>>
     getCursors(EvalState & state);
 
+    /* Get the first and most preferred cursor this Installable could refer
+       to, or throw an exception if none exists. */
     virtual ref<eval_cache::AttrCursor>
     getCursor(EvalState & state);
 
@@ -193,8 +197,8 @@ struct InstallableFlake : InstallableValue
 
     std::pair<Value *, PosIdx> toValue(EvalState & state) override;
 
-    /* Get a cursor to every attrpath in getActualAttrPaths() that
-       exists. */
+    /* Get a cursor to every attrpath in getActualAttrPaths()
+       that exists. However if none exists, throw an exception. */
     std::vector<ref<eval_cache::AttrCursor>>
     getCursors(EvalState & state) override;