libobs-d3d11: Avoid exceptions in shader cache file handling

[notr1ch]

Description

We're still getting crash reports from this code and it seems like C++ file I/O with exceptions set is a minefield - even closing a file in an exception handler can trigger further exceptions from buffer flushes for example. Using std::filesystem to check the file exists before opening it also introduces exceptions and is a pointless TOCTOU check anyway.

This commit removes the exception bits from the streams and relies on ifstream operator bool and ofstream fail() instead, greatly reducing the number of possible exception generating paths we need to worry about.

Motivation and Context

Fix reported crashes.

How Has This Been Tested?

Tested with read only, deleted files, corrupt files as much as possible.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.