htmlreport: fix #5998, crash on some files with "UnicodeDecodeError".

For those files that we can't decode, don't generate a html-preview but note into index.html that we couldn't process the file.
At the end, summarize on which files we failed and suggest using --source-encoding option.

htmlreport/cppcheck-htmlreport

@@ -308,6 +308,7 @@ if __name__ == '__main__':
     # Generate a HTML file with syntax highlighted source code for each
     # file that contains one or more errors.
     print('Processing errors')
+    decode_errors = []
     for filename, data in sorted(files.items()):
         htmlfile = data['htmlfile']
         errors = data['errors']
@@ -327,6 +328,11 @@ if __name__ == '__main__':
             sys.stderr.write("ERROR: Source file '%s' not found.\n" %
                              source_filename)
             continue
+        except UnicodeDecodeError:
+            sys.stderr.write("WARNING: Unicode decode error in '%s'.\n" %
+                             source_filename)
+            decode_errors.append(source_filename[2:]) # "[2:]" gets rid of "./" at beginning
+            continue
 
         htmlFormatter = AnnotateCodeFormatter(linenos=True,
                                               style='colorful',
@@ -369,6 +375,10 @@ if __name__ == '__main__':
         output_file.write(
             '       <tr><th>Line</th><th>Id</th><th>Severity</th><th>Message</th></tr>')
         for filename, data in sorted(files.items()):
+            if filename in decode_errors: # don't print a link but a note
+                output_file.write("\n       <tr><td colspan='4'>%s</td></tr>" % (filename))
+                output_file.write("\n       <tr><td colspan='4'> Could not generated due to UnicodeDecodeError</td></tr>")
+            else:
                 output_file.write(
                 "\n       <tr><td colspan='4'><a href='%s'>%s</a></td></tr>" %
                 (data['htmlfile'], filename))
@@ -393,9 +403,14 @@ if __name__ == '__main__':
                             (data['htmlfile'], error['line'], error['line'],
                              error['id'], error['severity'], error_class,
                              error['msg']))
+
         output_file.write('\n       </table>')
         output_file.write(HTML_FOOTER % contentHandler.versionCppcheck)
 
+        if (decode_errors):
+            sys.stderr.write("\nGenerating html failed for the following files: " + ' '.join(decode_errors))
+            sys.stderr.write("\nConsider changing source-encoding (for example: \"htmlreport ... --source-encoding=\"iso8859-1\"\"\n")
+
     print('Creating style.css file')
     with io.open(os.path.join(options.report_dir, 'style.css'),
                  'w') as css_file: