Improve error message for algo contruction on model with dangling edges

[ThijsvdLaar]

Defining an algorithm that requires a backward message on a dangling edge will throw a cryptic error. For example:

using ForneyLab
g = FactorGraph()
@RV x ~ GaussianMeanVariance(0.0, 1.0)
@RV y ~ GaussianMeanVariance(0.0, 1.0)
@RV z = x + y
sumProductAlgorithm(x)

will throw

KeyError: key nothing not found

Improving this error should enable a user to more quickly identify the source of the problem. This issue relates to #64 .