Allow throwing from exit handler

[ripopov]

For example for unit testing purposes we may want to replace SIGABRT with
throwing an exception, so that unit tests for CHECK()s can be written.

Solves #343

[KjellKod]

Throwing from a destructor is a big no-no in C++ which I didn’t think of immediately. I’ve to think on this one.

Regardless, please add unit test for this that has EXPECT_THROW
Just a pet peeve of mine. Your description isn’t accurate!
Writing unit tests with CHECK is possible without this. It’s just not possible at the moment to do it exactly the way you want to do it.

The way it was approached before is a safer way to do it than mucking with throwing destructors. This is clear in the issue you raised and can be seen throughout test_io.cpp

[ripopov]

Added unit test.

Throwing from a destructor is a big no-no in C++ which I didn’t think of immediately. I’ve to think on this one.

In general yes. However, this usage case, where we utilize destructor of temporary object to do logging, could be a legitimate exception of this rule.

[ripopov]

The way it was approached before is a safer way to do it than mucking with throwing destructors.

Don't agree. If invariant is broken, we should not continue. Like in unit test I've added, we should not allow code to continue and deference an invalid pointer.

[KjellKod]

LGTM, thanks for the contribution. It's cool to see different ways and approaches