Return a structured Image.Error from every open and write path

[hlindset]

• Every error path in Image.open/2 and Image.write/3 now returns {:error, %Image.Error{}}.
• :operation is now set for all open and write errors
• Fixes a bug where on error, Image.open!/2 would put the entire image into :path, and then interpolate it into :message
• Simplify Image.Error.wrap/2, delegating the work to Image.Error.exception/1 instead. This removes potential error message drift (which had already happened for :enoent)
• Remove Image.Error tuple constructors
• Rename internal vix_open/2 to open_path/2 for consistency with the new open_binary/2 and open_enum/2 functions

Breaking changes:

• Image.from_binary/2 returns {:error, %Image.Error{}} instead of {:error, binary}
• Image.write/3 to a Plug.Conn returns %Image.Error{reason: :closed} rather than {:error, :closed}
• Image.Error.exception(reason: :enoent, path: p) now has wrap/2's message, "The image file ... was not found or could not be opened", instead of "File not found: ..."
• A {atom, value} reason built through Image.Error.wrap/2 gains the "Image error: " prefix
• Internal:raise Image.Error, {:enoent, path} and raise Image.Error, {message, path} no longer have clauses and fall through to the catch-all (was documented behaviour)