Reduce messages for `bias_quantile()`

[nikosbosse]

Currently, bias_quantile() uses an internal helper function, bias_quantile_single_vector() to compute bias for a single forecast.

This helper functions throws a message if the median is not part of the quantile levels provided. That means that a single call to bias_quantile() can produce a lot of messages, as the message is repeated every time the helper is called. I.e. this:

observed <- c(1, -15, 22)
predicted <- rbind(
  c(-1, 0, 1, 2, 3),
  c(-2, 1, 2, 2, 4),
  c(-2, 0, 3, 3, 4)
)
quantile <- c(0.1, 0.25, 0.5, 0.75, 0.9)

bias_quantile(observed, predicted[, -3], quantile[-3])

produces three messages. I propose to move the message from the helper function to bias_quantile(), thus only getting one message for a single call to bias_quantile().