Bug 754921: improve messages with invalid syntax

[nickolay]

1. When JSON.parse() failed on invalid JSON (e.g. extraneous trailing
   comma) in ArgumentsJSON it raised a SyntaxError with no position
   information, which was displayed to the user as:

Syntax error at line , column : Unexpected token

2. Also before this change all invalid parameter syntax (such as
   {{macro(1,)}} matched the ArgumentsJSON production, thus hitting the bug
   (1) described above.

3. Also improved a helper in test-macros.js to help newbies (like
   myself) understand the structure of the testcase fixtures.

[nickolay]

So with these changes you get message like these:

Syntax error at line 4, column 10: Expected "'", "\"", [ \t\n\r;] or [\-.0-9] but ")" found.
    2 | produces a sensible error message.
    3 | 
    4 | {{ add(1,) }}
-----------------^


Syntax error at line 7, column 3: Expected valid JSON object as the parameter of the preceding macro but "{\n    \"es\":\"es/JavaScript/Acerca_de_JavaScript\",\n    \"en\": \"en/JavaScript/About_JavaScript\",\n}" found.
    5 |     "es":"es/JavaScript/Acerca_de_JavaScript",
    6 |     "en": "en/JavaScript/About_JavaScript",
    7 | }) }}
----------^

You get even better error messages when defining the JSON grammar with PEG (by inlining https://github.com/dmajda/pegjs/blob/master/examples/json.pegjs ), but unfortunately

• pegjs doesn't support including one grammar in another one (so we'd have to fork json.pegjs and modify it a little (|start|, |whitespace|))
• json.pegjs has a FIXME about not supporting null values, which I'm not sure is acceptable for us.
• the fix here is less regression-prone, since it (mostly) only affects failure cases.

[lmorchard]

Tests pass, error messages improved in a manual test, hooray! Thanks for tracking that down!