Add warning if problematic --tesseract-pagesegmode is selected

Fixes #549

src/ocrmypdf/_validation.py

@@ -276,6 +276,11 @@ def check_options_advanced(options):
             "Tesseract 4.0 ignores --user-words and --user-patterns, so these "
             "arguments have no effect."
         )
+    if options.tesseract_pagesegmode in (0, 2):
+        log.warning(
+            "The --tesseract-pagesegmode argument you select will disable OCR. "
+            "This may cause processing to fail."
+        )
 
 
 def check_options_metadata(options):

tests/test_validation.py

@@ -210,3 +210,9 @@ def test_version_comparison():
             version_checker=lambda: '1.0',
             need_version='2.0',
         )
+
+
+def test_pagesegmode_warning(caplog):
+    opts = make_opts(tesseract_pagesegmode='0')
+    vd.check_options_advanced(opts)
+    assert 'disable OCR' in caplog.text