Fail if metadata.json contains invalid escape sequence

This is an enhancement of voxpupuli#120

We now use a regex to identify invalid escape sequences. `JSON.parse()`
has no built-in way to detect this. Even in strict mode it ignores it:

```
irb(main):002> require 'json'
=> true
irb(main):003> JSON.parser = JSON::Ext::Parser
=> JSON::Ext::Parser
irb(main):004> JSON.parse('{"summary": "A description with an invalid \( escape sequence"}')
=> {"summary"=>"A description with an invalid ( escape sequence"}
irb(main):005> JSON.parse("{\"summary\": \"A description with an invalid \( escape sequence\"}")
=> {"summary"=>"A description with an invalid ( escape sequence"}
irb(main):006>
```

Author: bastelfreak

lib/metadata_json_lint.rb

@@ -69,6 +69,14 @@ def run
   end
   module_function :run
 
+  def contains_invalid_escape?(content)
+    # Regex looks for:
+    # 1. Invalid escape sequences (\x or incomplete \u)
+    invalid_escape_regex = %r{\\[^"/bfnrtu]|\\u(?![0-9a-fA-F]{4})}
+    content.match?(invalid_escape_regex)
+  end
+  module_function :contains_invalid_escape?
+
   def parse(metadata)
     @errors = []
     @warnings = []
@@ -83,6 +91,8 @@ def parse(metadata)
       abort("Error: Unable to read metadata file: #{e.exception}")
     end
 
+    abort('Error: Unable to parse metadata.json: Invalid escape character in string') if contains_invalid_escape?(f)
+
     begin
       parsed = JSON.parse(f)
     rescue Exception => e