Find abstraction for SourceException in TripProbabilities

[t-ober]

    There's many copies of almost exactly this code in here, maybe we can find an abstraction for this e.g. in form of a common method

Originally posted by @sebastian-peter in #79 (comment)

[t-ober]

"One suggestion would be to actually throw the exceptions from within the getFromFile methods, so that we can catch them here and re-throw them within a SourceException. Pretty old school, but effective. Could use checked exceptions if one is concerned about safety here." - @sebastian-peter

[t-ober]

Referenced code example

case Failure(exception) =>
        throw SourceException(
          "Unable to get probabilities for first departure of day from path.",
          exception
        )
      case Success(value) => value
    }

[sebastian-peter]

Ok we can still keep the functional util.Try by e.g. using a for comprehension:

for {
  a <- getA()
  b <- getB()
} yield (a, b)

This way, execution stops with the first failure and a Failure(ex) is returned, otherwise we get Success((a,b))