Error if file listed in config.yml content section does not exist

(refs #9638)

python/MooseDocs/common/get_content.py

@@ -10,6 +10,7 @@
 #pylint: enable=missing-docstring
 import os
 import re
+import copy
 import logging
 import mooseutils
 import MooseDocs
@@ -106,6 +107,12 @@ def _doc_import(root_dir, content=None):
     for pattern in exclude:
         output -= _find_files(output, os.path.join(root_dir, pattern))
 
+    # Test the files exist
+    for fname in copy.copy(output):
+        if not os.path.isfile(fname):
+            LOG.error("Unknown file provided in content (it is being removed):\n{}".format(fname))
+            output.remove(fname)
+
     return sorted(output)
 
 def create_file_page(name, filename, in_ext):