tests: refactor VppDiedError.

tests: refactor VppDiedError.

- Move Exception into same module as TestCase.
- Move the error reporting logic inside the error.
- Allows testing of the returncode and signal_name for tests to consume.
- Fix the signal reporting code:
	VppDiedError: VPP subprocess died unexpectedly with returncode -6 [unknown].
    displays as:
	VppDiedError: VPP subprocess died unexpectedly with return code: -6 [SIGABRT].

Type: test

Change-Id: I8488ab318a596c9b737308829cedfb7e96e57302
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>