Fix for KeyError: 'region', caused by result without region

Region / line / column information are present in most messages.  The one that
caused this error refers to the whole file:

    ipdb> p sarif_struct

    {'ruleId': 'com.lgtm/cpp-queries:cpp/missing-header-guard', 'ruleIndex': 12,
    'message': {'text': 'This header file should contain a header guard to prevent
    multiple inclusion.'}, 'locations': [{'physicalLocation': {'artifactLocation':
    {'uri': 'diff/cmpbuf.h', 'uriBaseId': '%SRCROOT%', 'index': 13}}}],
    'partialFingerprints': {'primaryLocationLineHash': 'd04cb834fa64727d:1',
    'primaryLocationStartColumnFingerprint': '0'}}

The goal is fixed-structure output formatting, so whole-file output uses
-1,-1,-1,-1 for line, column information.

sarif_cli/__init__.py

@@ -33,14 +33,22 @@ def get_relatedlocation_message_info(related_location):
     region = get(related_location, 'physicalLocation', 'region')
     return message, artifact, region
 
+class WholeFile:
+    pass
+
 def get_location_message_info(result):
     """ Given one of the results, extract message information.
 
     The `result` typically starts from get(sarif_struct, 'runs', run_index, 'results', res_index)
+
+    Returns: (message, artifact, region)
+        For an empty 'region' key, returns (message, artifact, sarif_cli.WholeFile)
+
     """
     message = get(result, 'message', 'text')
     artifact = get(result, 'locations', 0, 'physicalLocation', 'artifactLocation')
-    region = get(result,  'locations', 0, 'physicalLocation', 'region')
+    # If there is no 'region' key, use the whole file
+    region = get(result,  'locations', 0, 'physicalLocation').get('region', WholeFile)
     return (message, artifact, region)
 
 def display_underlined(l1, c1, l2, c2, line, line_num):