Moved all custom exceptions to excptions.py

src/binwalk/core/exceptions.py

+class ParserException(Exception):
+    '''
+    Exception thrown specifically for signature file parsing errors.
+    '''
+    pass
+
+class ModuleException(Exception):
+    '''
+    Module exception class.
+    Nothing special here except the name.
+    '''
+    pass
+
+class IgnoreFileException(Exception):
+    '''
+    Special exception class used by the load_file plugin method
+    to indicate that the file that we are attempting to load
+    should be ignored.
+    '''
+    pass