Add logic to print line number of JSON errors

This commit makes the pretty JSON check more verbose when it encounters
errors, that way developers can see which lines are causing errors in
order to debug.

pre_commit_hooks/pretty_format_json.py

diff --git a/pre_commit_hooks/pretty_format_json.py b/pre_commit_hooks/pretty_format_json.py
index e734ca8..f42630a 100644
--- a/pre_commit_hooks/pretty_format_json.py
+++ b/pre_commit_hooks/pretty_format_json.py
@@ -115,6 +115,17 @@
             if contents != pretty_contents:
                 print('File {} is not pretty-formatted'.format(json_file))
 
+                contents_by_line = ''.join(contents).split('\n')
+                pretty_contents_by_line = ''.join(pretty_contents).split('\n')
+
+                diff = len(contents_by_line) - len(pretty_contents_by_line)
+                if diff > 0:
+                    pretty_contents_by_line.extend([''] * diff)
+
+                for line_num, line in enumerate(contents_by_line):
+                    if line != pretty_contents_by_line[line_num]:
+                        print('{}:{}'.format(json_file, line_num))
+
                 if args.autofix:
                     _autofix(json_file, pretty_contents)