Skip duplicated error message when omitting the traceback

The new BuildError exception is good for skipping the traceback, but if it's skipped, now the two copies of the error mesage (one from logging, one from reaching SystemExit) immediately follow each other, which looks strange.
So, skip logging the exception's message when encountering a BuildError in _populate_page and _build_page.

mkdocs/commands/build.py

@@ -179,7 +179,11 @@ def _populate_page(page, config, files, dirty=False):
             'page_content', page.content, page=page, config=config, files=files
         )
     except Exception as e:
-        log.error("Error reading page '{}': {}".format(page.file.src_path, e))
+        message = f"Error reading page '{page.file.src_path}':"
+        # Prevent duplicated the error message because it will be printed immediately afterwards.
+        if not isinstance(e, BuildError):
+            message += f" {e}"
+        log.error(message)
         raise
 
 
@@ -227,7 +231,11 @@ def _build_page(page, config, doc_files, nav, env, dirty=False):
         # Deactivate page
         page.active = False
     except Exception as e:
-        log.error("Error building page '{}': {}".format(page.file.src_path, e))
+        message = f"Error building page '{page.file.src_path}':"
+        # Prevent duplicated the error message because it will be printed immediately afterwards.
+        if not isinstance(e, BuildError):
+            message += f" {e}"
+        log.error(message)
         raise
 
 

mkdocs/tests/build_tests.py

@@ -3,6 +3,7 @@
 from unittest import mock
 import unittest
 
+from mkdocs.exceptions import PluginError
 from mkdocs.structure.pages import Page
 from mkdocs.structure.files import File, Files
 from mkdocs.structure.nav import get_navigation
@@ -334,6 +335,21 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
         )
         self.assert_mock_called_once(mock_open)
 
+    @tempdir(files={'index.md': 'page content'})
+    @mock.patch('mkdocs.plugins.PluginCollection.run_event', side_effect=PluginError('Error message.'))
+    def test_populate_page_read_plugin_error(self, docs_dir, mock_open):
+        cfg = load_config(docs_dir=docs_dir)
+        file = File('index.md', cfg['docs_dir'], cfg['site_dir'], cfg['use_directory_urls'])
+        page = Page('Foo', file, cfg)
+        with self.assertLogs('mkdocs', level='ERROR') as cm:
+            self.assertRaises(PluginError, build._populate_page, page, cfg, Files([file]))
+        self.assertEqual(
+            cm.output, [
+                "ERROR:mkdocs.commands.build:Error reading page 'index.md':"
+            ]
+        )
+        self.assert_mock_called_once(mock_open)
+
     # Test build._build_page
 
     @tempdir()
@@ -433,6 +449,25 @@ class BuildTests(PathAssertionMixin, unittest.TestCase):
         )
         self.assert_mock_called_once(mock_write_file)
 
+    @tempdir()
+    @mock.patch('mkdocs.plugins.PluginCollection.run_event', side_effect=PluginError('Error message.'))
+    def test_build_page_plugin_error(self, site_dir, mock_write_file):
+        cfg = load_config(site_dir=site_dir, nav=['index.md'], plugins=[])
+        files = Files([File('index.md', cfg['docs_dir'], cfg['site_dir'], cfg['use_directory_urls'])])
+        nav = get_navigation(files, cfg)
+        page = files.documentation_pages()[0].page
+        # Fake populate page
+        page.title = 'Title'
+        page.markdown = 'page content'
+        page.content = '<p>page content</p>'
+        with self.assertLogs('mkdocs', level='ERROR') as cm:
+            self.assertRaises(PluginError, build._build_page, page, cfg, files, nav, cfg['theme'].get_env())
+        self.assertEqual(
+            cm.output,
+            ["ERROR:mkdocs.commands.build:Error building page 'index.md':"]
+        )
+        self.assert_mock_called_once(mock_write_file)
+
     # Test build.build
 
     @tempdir(files={