Fix JsonParser error behavior

[resnickj]

Issues:

• JsonParser sometimes throws JavaScriptException (SyntaxError) and sometimes throws ParserException; should be consistent and always throw one or the other.
• A script that calls JSON.parse(input) should always see a SyntaxError in the case of syntactically invalid input.
• SyntaxError messages should clearly indicate that the error is in JSON data, and indicate the position within the input string.
• SyntaxError messages should not include the entire input string in the message, as it may be arbitrarily long.

Changes:

• Route all exceptions through a common ThrowError(...) method that always throws a SyntaxError
• Clean-up content of error messages and include position of the error. Unfortunately the parser does not track line number accurately, so position is rendered as a single number as opposed to line:column format.
• Add tests to verify the expected error output for each call site of ThrowError(...)

[lahma]

Great work, thank you!