Return an ok/error tuple from Image.average/1 and Image.chroma_color/1

[hlindset]

Image.average/1 returned a bare list on success and {:error, reason} on failure, which made it awkward to use in with/case statements, having to guard on is_list/1 or rely on clause ordering instead of matching on an :ok/:error tuple.

• The declared success type was wider than the actual return: Corrected from Pixel.t() to [number()] (and now wrapped in an :ok tuple)
• Errors are now wrapped in an Image.Error struct
• Image.chroma_color/1 now returns {:ok, [number()]} | {:error, Image.Error.t()} since it returns the result of Image.average/1
• Added Image.chroma_color!/1 as a companion to Image.chroma_color/1
• Added a test exercising errors in average/1 (using a truncated JPEG)
• chroma_mask/2 and find_trim/2 now propagate a failed :auto color lookup instead of using the error tuple as a pixel

Breaking changes

Image.average/1 returns {:ok, [number()]} | {:error, Image.Error.t()}

# Before
color = Image.average(image)

# After
{:ok, color} = Image.average(image)

• Code that guarded on shape (when is_list(color)) should now match {:ok, color} instead
• Code that relied on clause ordering can now match {:ok, color} and {:error, reason} in any order
• Errors are now Image.Error structs instead of binary()

Image.chroma_color/1 returns {:ok, [number()]} | {:error, Image.Error.t()}

# Before
color = Image.chroma_color(image)

# After
{:ok, color} = Image.chroma_color(image)

Previously the spec claimed Pixel.t() with no error case, which was inaccurate, as all of flatten/2, crop/5 and average/1 could return an error tuple.