Minor improvement to XHPASTSyntaxErrorException message

Summary: Trim trailing newline from exception message.

Test Plan:
**Before**
```
> ./scripts/phutil_symbols.php test.php
{
  "error" : "XHPAST Parse Error: syntax error, unexpected $end, expecting T_FUNCTION on line 4\n",
  "line"  : "4",
  "file"  : "\/home\/joshua\/workspace\/github.com\/phacility\/libphutil\/foo.php"
}
```

**After**
```
> ./scripts/phutil_symbols.php test.php
{
  "error" : "XHPAST Parse Error: syntax error, unexpected $end, expecting T_FUNCTION on line 4",
  "line"  : "4",
  "file"  : "\/home\/joshua\/workspace\/github.com\/phacility\/libphutil\/foo.php"
}
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9961