BUGFIX: warning when png

When adding a `png` the two cache calls throw warnings currently

I'm converting all errors/warnings to exception
```
2014-11-03T10:40:12+01:00 WARN (4): exception 'ErrorException' with message 'getimagesize(....\data\cache\tcpdf__tcpdf_imgmask_alpha_4a927e302242781011473e99c88d536b): failed to open stream: No such file or directory, URI: /changeover/report/general, Referrer: http://.../changeover/product/select, Username: degjea' in ...\vendor\tecnick.com\tcpdf\tcpdf.php:6882
Stack trace:
#0 [internal function]: Logger\Logger::Logger\{closure}(2, 'getimagesize(D:...', 'D:\Zend\Apache2...', 6882, Array)
#1 ...\vendor\tecnick.com\tcpdf\tcpdf.php(6882): getimagesize('D:\Zend\Apache2...')
#2 ...\vendor\tecnick.com\tcpdf\tcpdf.php(7035): TCPDF->Image('D:\Zend\Apache2...', 10, 105.760416, 6, 6, 'PNG', '', '', false, 300, '', true, false)

```
This doesn't work because getimagesize works also with URLs, wile file_exists works only locally.