check if blast completed successfully before clustering

[sestaton]

This check should be done in the pair finding step prior to clustering, and is an easy fix that would simplify debugging. Tests should be added also to make sure the program exits as expected.

[sestaton]

A method was added to the Transposome::Pairfinder class in cf4c735 to handle empty blast input, and a check was added to the main application in c6db944 so the entire process does not try to proceed.

Tests for object construction were added in b81e4b4.