Set error variable if there were some errors during refinement

[StefanBruens]

When as_pool_refine_data encounters bad components, this is reflected
in the return code, but there is no further information given what went
wrong. Give a short hint to the user, and refer to the debug output which
allows determining the cause.

[ximion]

This makes sense. Thank you for the patch!

[ximion]

Looks like I was a bit too quick with merging this:

../src/as-pool.c:1020:12: error: implicit declaration of function 'g_new_error_literal'; did you mean 'g_set_error_literal'? [-Werror=implicit-function-declaration]
   *error = g_new_error_literal(AS_POOL_ERROR,

[StefanBruens]

oops - g_error_new_literal ...