Fix ImageAssert.read to compile successfully against Java 1.5

The current sourcecode uses a IOException constructor which is only available in Java 1.6 and later:

new IOException(concat("Unable to read image from file ", quote(imageFilePath)), e);

This code won't compile against Java 1.5, which is the Java version we chose as our source and target platform in the parent pom.xml

Fix this by not catching the IOException. Instead, simply let it fly "as is".