Set file.path as the error object's .fileName

The default error message generated when there's a syntax error in a JSX file doesn't include the source file name, only the line number the error was found on.

Adding a filePath property to the error object exposes this information for use in error handlers, e.g.:

```javascript
var plumber = require('gulp-plumber')
var react = require('gulp-react')

gulp.task('compile-jsx', function() {
  return gulp.src('./src/**/*.jsx')
    .pipe(plumber())
    .pipe(react())
    .on('error', function(e) {
      console.error(e.message + '\n  in ' + e.fileName)
    })
    .pipe(gulp.dest('./build/modules'))
})
```