Fix "clobbered variable" compiler warning (-Wclobbered)

[6arms1leg]

This PR adds a missing volatile type qualifier to fix a "clobbered variable" compiler warning (-Wclobbered).

Compiling the unit tests with the -Wclobbered compiler flag (or -Wextra, which includes -Wclobbered) produces the following warning:

warning: variable ‘e’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Wclobbered]

The CExeption README states that

If (a) you change local (stack) variables within your Try block, and (b) wish to make use of the updated values after an exception is thrown, those variables should be made volatile.

I believe both conditions apply to the variable I added the volatile type qualifier to.
This is supported by this SO answer and this SO answer.

Now, the compiler issues no warning when compiled with the -Wclobbered or -Wextra flag.