message: techdocs: don't swallow errors other than NotModifiedError
diff: * See the License for the specific language governing permissions and
  * limitations under the License.
  */
  +import { NotModifiedError } from '@backstage/backend-common';
  import { Entity } from '@backstage/catalog-model';
  import {
  GeneratorBase,
  @@ -98,10 +99,13 @@ export class DocsBuilder {
  preparedDir = preparerResponse.preparedDir;
  etag = preparerResponse.etag;
  - } catch (NotModifiedError) {
  + } catch (err) {
  + if (err instanceof NotModifiedError) {
  // No need to prepare anymore since cache is valid.
  return;
  }
  + throw new Error(err.message);
  + }
  this.logger.info(
  `TechDocs prepare step completed for entity ${getEntityId(this.entity)}.`,
  