Clarify errors in scheduler docstrings (#488)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

Minor clarifications to how the error messages work in the scheduler. In
ask(), we aren't checking if tell() is called; we're just checking if
another ask method was previously called.

## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [x] I have added a one-line description of my change to the changelog
in
      `HISTORY.md`
- [x] This PR is ready to go

HISTORY.md

@@ -23,6 +23,7 @@
 - Switch from std to var in arm tutorial ({pr}`486`)
 - Fix documentation conf for readthedocs deprecations ({pr}`485`)
 - Add novelty search with CMA-ES to sphere example ({pr}`478`, {pr}`482`)
+- Clarify errors in scheduler docstrings ({pr}`488`)
 
 #### Improvements
 

ribs/schedulers/_bandit_scheduler.py

@@ -217,8 +217,8 @@ def ask(self):
             (batch_size, dim) array: An array of n solutions to evaluate. Each
             row contains a single solution.
         Raises:
-            RuntimeError: This method was called without first calling
-                :meth:`tell`.
+            RuntimeError: This method was called immediately after calling an
+                ask method.
         """
         if self._last_called == "ask":
             raise RuntimeError("ask cannot be called immediately after " +
@@ -331,7 +331,7 @@ def tell(self, objective, measures, **fields):
                 argument should be an array with batch_size as the first
                 dimension.
         Raises:
-            RuntimeError: This method is called without first calling
+            RuntimeError: This method was called without first calling
                 :meth:`ask`.
             ValueError: One of the inputs has the wrong shape.
         """

ribs/schedulers/_scheduler.py

@@ -154,8 +154,8 @@ def ask_dqd(self):
             (batch_size, dim) array: An array of n solutions to evaluate. Each
             row contains a single solution.
         Raises:
-            RuntimeError: This method was called without first calling
-                :meth:`tell`.
+            RuntimeError: This method was called immediately after calling an
+                ask method.
         """
         if self._last_called in ["ask", "ask_dqd"]:
             raise RuntimeError("ask_dqd cannot be called immediately after " +
@@ -185,8 +185,8 @@ def ask(self):
             (batch_size, dim) array: An array of n solutions to evaluate. Each
             row contains a single solution.
         Raises:
-            RuntimeError: This method was called without first calling
-                :meth:`tell`.
+            RuntimeError: This method was called immediately after calling an
+                ask method.
         """
         if self._last_called in ["ask", "ask_dqd"]:
             raise RuntimeError("ask cannot be called immediately after " +
@@ -307,8 +307,8 @@ def tell_dqd(self, objective, measures, jacobian, **fields):
                 argument should be an array with batch_size as the first
                 dimension.
         Raises:
-            RuntimeError: This method is called without first calling
-                :meth:`ask`.
+            RuntimeError: This method was called without first calling
+                :meth:`ask_dqd`.
             ValueError: One of the inputs has the wrong shape.
         """
         if self._last_called != "ask_dqd":
@@ -362,7 +362,7 @@ def tell(self, objective, measures, **fields):
                 argument should be an array with batch_size as the first
                 dimension.
         Raises:
-            RuntimeError: This method is called without first calling
+            RuntimeError: This method was called without first calling
                 :meth:`ask`.
             ValueError: One of the inputs has the wrong shape.
         """