Avoid C4702 "unreachable code" in MSVC release mode.

[Sebanisu]

Problem:

• compiler-warning-level-4-c4702 would trigger on the default/second return statement in MSVC when compiling in release mode. At first I tried just moving the return statement to inside the try block, but the error still occurred.

Solution:

• Single return statement using a bool variable that gets toggled to true/false depending on if any catch statements are ran.

fixes: https://github.com/boost-ext/ut/issues/443

Reviewers: @