Check errors from Close method in SavePNG and SaveJPG

[peterstace]

When closing the file, errors can still occur that might not be surfaced during the writes from png.Encode and jpeg.Encode. By returning the error from file.Close(), these cases can be caught.

[sbinet]

FYI, something along these lines has been done in "my" fork of gg:

• https://sr.ht/~sbinet/gg/