Throw an exception if the field doesn't exist in Schema::dropField

Problem/Motivation

If you see the first lines of the Schema::dropField() method you will see that if there is not field no exception is thrown.

    if (!$this->fieldExists($table, $field)) {
      return FALSE;
    }

Proposed resolution

Throw an exception like in the Schema::addField() method

    if (!$this->tableExists($table)) {
      throw new SchemaObjectDoesNotExistException(t("Cannot add field @table.@field: table doesn't exist.", ['@field' => $field, '@table' => $table]));
    }

But with the correct message.

Remaining tasks

Test the patch.

User interface changes

None.

API changes

Update the doc with the throw exception info.

Data model changes

None.