Ignore EXIF exceptions

Backporting a bug fix from the official Sorl Thumbnail library, to this custom version.

The modification I am backporting was made in this commit:
https://github.com/mariocesar/sorl-thumbnail/commit/66a371d14a51526df7cbcef9cc61823f265a44b8
Line 58

Discussed in issue 98:
https://github.com/mariocesar/sorl-thumbnail/issues/98