Make it easier to instantiate ProblemDetails

[jsha]

Right now we have a common pattern:

return &probs.ProblemDetails{
  Type: probs.ConnectionProblem,
  Detail: err.Error(),
}

This is unnecessarily verbose and makes it harder to read the surrounding code. Instead, we should turn each of the probs.*Problem variables from ProblemTypes into functions, and say:

return probs.ConnectionProblem(err)

[cpu]

I like the idea of having probs.*Problem functions to quickly construct a ProblemDetails of the right type. It looks like some of the ProblemTypes have functions like this already (e.g. BadNonceProblem has BadNonce).

I will add one for ConnectionProblem and convert callers using the explicit ProblemDetails struct instantiation.