Include exception message from Spree::Core::GatewayErrors

This makes the error messages when payments fail slightly less vague than they used to be.

Fixes #3851
Bump activemerchant to 1.39.2

Conflicts:
	core/spree_core.gemspec
Merge branch 'source/2-0-stable' into 2-0-stable

# By Ryan Bigg
# Via Ryan Bigg
* source/2-0-stable:
  Bump activemerchant to 1.39.2
  Include exception message from Spree::Core::GatewayErrors

Conflicts:
	core/spree_core.gemspec