Offer hint to user to use --max-image-mpixels after decompression bob error

Closes #801

src/ocrmypdf/_sync.py

@@ -414,6 +414,13 @@ def run_pipeline(options, *, plugin_manager, api=False):
         else:
             log.error(type(e).__name__)
         return e.exit_code
+    except (PIL.Image.DecompressionBombError if not api else NeverRaise) as e:
+        log.exception(
+            "A decompression bomb error was encountered while executing the "
+            "pipeline. Use the argument --max-image-mpixels to raise the maximum "
+            "image pixel limit."
+        )
+        return ExitCode.other_error
     except (Exception if not api else NeverRaise):  # pylint: disable=broad-except
         log.exception("An exception occurred while executing the pipeline")
         return ExitCode.other_error

tests/test_main.py

@@ -745,14 +745,13 @@ def test_pdfa_n(pdfa_level, resources, outpdf):
     assert pdfa_info['conformance'] == f'PDF/A-{pdfa_level}B'
 
 
-@pytest.mark.skipif(
-    PIL.__version__ < '5.0.0', reason="Pillow < 5.0.0 doesn't raise the exception"
-)
-@pytest.mark.slow
-def test_decompression_bomb(resources, outpdf):
+def test_decompression_bomb_error(resources, outpdf):
     p, _out, err = run_ocrmypdf(resources / 'hugemono.pdf', outpdf)
-    assert 'decompression bomb' in err
+    assert 'decompression bomb' in err and '--max-image-mpixels' in err
 
+
+@pytest.mark.slow
+def test_decompression_bomb_succeeds(resources, outpdf):
     p, _out, err = run_ocrmypdf(
         resources / 'hugemono.pdf', outpdf, '--max-image-mpixels', '2000'
     )