(core) Show count of formula errors in the column config in the right-side panel.

Summary:
- Cache the count by column, factoring out ColumnCache from
  ColumnACIndexes, which uses a similar pattern.
- Update error counts in response to column selection and to data changes.

Test Plan: Adds a test case for the new message

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2780

app/client/models/TableData.ts

@@ -2,8 +2,11 @@
  * TableData maintains a single table's data.
  */
 import {ColumnACIndexes} from 'app/client/models/ColumnACIndexes';
+import {ColumnCache} from 'app/client/models/ColumnCache';
 import {DocData} from 'app/client/models/DocData';
 import {DocAction, ReplaceTableData, TableDataAction, UserAction} from 'app/common/DocActions';
+import {isRaisedException} from 'app/common/gristTypes';
+import {countIf} from 'app/common/gutil';
 import {ColTypeMap, TableData as BaseTableData} from 'app/common/TableData';
 import {BaseFormatter} from 'app/common/ValueFormatter';
 import {Emitter} from 'grainjs';
@@ -19,6 +22,8 @@ export class TableData extends BaseTableData {
 
   public readonly columnACIndexes = new ColumnACIndexes(this);
 
+  private _columnErrorCounts = new ColumnCache<number|undefined>(this);
+
   /**
    * Constructor for TableData.
    * @param {DocData} docData: The root DocData object for this document.
@@ -92,6 +97,17 @@ export class TableData extends BaseTableData {
     return ret;
   }
 
+  /**
+   * Counts and returns the number of error values in the given column. The count is cached to
+   * keep it faster for large tables, and the cache is cleared as needed on changes to the table.
+   */
+  public countErrors(colId: string): number|undefined {
+    return this._columnErrorCounts.getValue(colId, () => {
+      const values = this.getColValues(colId);
+      return values && countIf(values, isRaisedException);
+    });
+  }
+
   /**
    * Sends an array of table-specific action to the server to be applied. The tableId should be
    * omitted from each `action` parameter and will be inserted automatically.