Add errorBuilder to SvgPicture and adjust underlying vector_graphics errorBuilder (to trigger on BytesLoader errors)

[Yagorundy]

My use case: I am trying to fallback to a svg in assets if my SvgPicture.network() fails for any reason (wrong url, request returns 404, invalid svg format, ...)

widgets like the Flutter Image fully support this errorBuilder and any error in the process (image loading/parsing/rendering) triggers the errorBuilder callback

From my observations:

• SvgPicture widget does not accept errorBuilder
• The vector_graphics package does not trigger errorBuilder for BytesLoader objects loadBytes() calls, it only triggers it for errors in decodeVectorGraphics()

image from vector_graphics.dart

[guopeng1994]

WE NEED THIS!!

[mnayef95]

I've created a PR on vector_graphics. Once @dnfield approves it, we can update flutter_svg to handle it.

dnfield/vector_graphics#255