Don't stop optimization when pngquant cannot optimize

[brianpkennedy]

I am using v1.0.2. I assumed that since the arguments to pngquant include --skip-if-larger, that an image that failed to optimize because the outputted file was too large.

This is the output i get when using imagemin-pngquant through a grunt command, with verbose turned on.

Warning: Command failed: /var/folders/vk/vl8x2sx57mxg5skf6nr7lztn8r0910/T/25c835bd-fd08-418a-b752-ba5fd495cf45:
  read 5KB file
  made histogram...254 colors found
  eliminated opaque tRNS-chunk entries...158 entries transparent
  mapped image to new colors...MSE=0.000 (Q=100)
  writing 254-color image as 7a701b63-b8c4-48c0-a770-ad3bb876fe05
  file exceeded expected size of 4KB
  writing truecolor image as 7a701b63-b8c4-48c0-a770-ad3bb876fe05
Skipped 1 file out of a total of 1 file.
 Use --force to continue.

The call to the pngquant binary is returning an error code of 98, which is TOO_LARGE_FILE. Since we are skipping large files, this response should not be treated as an error.