[visualize] User-friendly error message for widthless/heightless layers

[jbinney]

If you attempt to visualize a view or layer with 0 width or height, you get a pretty inscrutable error message:

(lldb) visualize 0x7fa1285024b0
Nov  1 13:52:00  ChiselTestApp[87971] <Error>: CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Nov  1 13:52:00  ChiselTestApp[87971] <Error>: CGContextSaveGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Nov  1 13:52:00  ChiselTestApp[87971] <Error>: CGContextSetFillColorWithColor: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Nov  1 13:52:00  ChiselTestApp[87971] <Error>: CGContextAddRect: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Nov  1 13:52:00  ChiselTestApp[87971] <Error>: CGContextDrawPath: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Nov  1 13:52:00  ChiselTestApp[87971] <Error>: CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Nov  1 13:52:00  ChiselTestApp[87971] <Error>: CGContextRestoreGState: invalid context 0x0. If you want to see the backtrace, please set CG_CONTEXT_SHOW_BACKTRACE environmental variable.
Could not get image data.

With this change, we proactively generate a more user-friendly error message when the width or height of a layer is 0:

(lldb) visualize 0x7ff3fca37030
Nothing to see here - the size of this element is 0.0 x 1.5.

[arigrant]

This is great. Thanks!