Make invalid k-means options an %Image.Error{} + return {:ok, model} from Image.Scholar.k_means/2

[hlindset]

Image.k_means/2 and Image.reduce_colors/2 both pass options straight to Scholar.Cluster.KMeans.fit/2, which validates with NimbleOptions.validate!/2 and raises.

Image.k_means(img, num_clusters: 0)
# ** (NimbleOptions.ValidationError) invalid value for :num_clusters option: expected positive integer, got: 0

Changes

• Added Image.Scholar.fit/2 that rescues and translates NimbleOptions.ValidationError into %Image.Error{reason: :invalid_option}
• Made Image.reduce_colors/2 and Image.Scholar.k_means/2 both go through the new Image.Scholar.fit/2
• Moved the scholar call in Image.k_means/2 into the with to properly propagate errors

Breaking changes

Affects Image.k_means/2, Image.reduce_colors/2, Image.Scholar.k_means/2 and the bang variants.

• Invalid or unknown options return {:error, %Image.Error{reason: :invalid_option}}. Previously raised NimbleOptions.ValidationError
• The bang variants raise Image.Error. Previously raised NimbleOptions.ValidationError
• Image.Scholar.k_means/2 now returns {:ok, %Scholar.Cluster.KMeans{}}. Previously returned a bare %Scholar.Cluster.KMeans{} on success