Improve parsing error messages

[chqrlie]

• output more informative error messages in js_parse_expect.

The previous code was bogus:

    return js_parse_error(s, "expecting '%c'", tok);

this was causing a bug on eval("do;") where tok is TOK_WHILE (-70, 0xBA) creating an invalid UTF-8 encoding (lone trailing byte). This would ultimately have caused a failure in JS_ThrowError2 if JS_NewString failed when converting an error to a string if the conversion detects invalid UTF-8 encoding and throws an error (it currently does not, but should).

• test for JS_NewString failure in JS_ThrowError2
• test for JS_FreeCString failure in run-test262.c
• add more test cases