assert: improve AssertionError in case of "Errors"

Showing the stack trace in a error message obfuscates the actual message and should not be visible therefore.

I think this is actually part of the assert subsystem even though the AssertionError is placed in the internal/errors.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

assert