fix: improve error message in case of file not found

[zonca]

Currently if a file is missing, the error message is a C++ I/O exception:

In [6]: import awkward_kaitai

In [7]:     reader = awkward_kaitai.Reader("test_artifacts/libpixie4e.so")

In [8]:     awkward_array = reader.load("example_data/data/pixie4e.raw")
   ...: 
terminate called after throwing an instance of 'std::__ios_failure'
  what():  basic_ios::clear: iostream error
Aborted

Not urgent, but we could catch this in Python and give a better error message.

[ManasviGoyal]

@zonca Yes, that would be great. Thanks!