minimum viable depsolving exception handling fix

If you typo the "chef-client" cookbook as "chef_client" this code
produces:

  Unable to satisfy constraints on package chef_client, which does not
  exist, due to solution constraint (sk_role_base = 0.0.16). Solution
  constraints that may result in a constraint on chef_client:
  [(sk_role_base = 0.0.16) -> (chef_client >= 0.0.0)]
  Missing artifacts: chef_client
  Demand that cannot be met: (sk_role_base = 0.0.16)
  Unable to find a solution for demands: sk_role_base (0.0.16)

Previous code only prints:

  Unable to find a solution for demands: sk_role_base (0.0.16)

Which is useless to figure out you need to fix.
Merge pull request #1136 from lamont-granquist/lcg/patch-error-handling

Handle exceptions from dep-selector