[hotfix] Adapt exception class for const correctness

[jsturdy]

Description

When migrating functionality in cmsgemos, catching the exceptions raised by xhal by const reference was breaking const-correctness semantics.
The implemented what() in xhal was not const, as it is in the std::exception class it inherits from.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and Context

How Has This Been Tested?

I tested that ctp7_modules still compiles (but it doesn't care about much from xhal), otherwise, I am not sure what else might be affected by this. (I would say nothing should be, as this change just enforces a contract that was probably already made).