Handle YAML load errors from the new Psych YAML parser.

Earlier versions of Ruby used the syck library to parse YAML; during 1.9 this
moved to the Psych parser based on libyaml. This brings YAML 1.1 compliance
to the library, and generally moves in a forward direction.

It also makes some radical changes to the failure modes, since Psych is much
more forgiving of some YAML constructs that would previously block the heck
up.

It also, in Ruby tradition, does absolutely nothing to abstract over the
implementation: Ruby can use either Psych or Syck and will raise different
exceptions on failure in both cases.

Worse, Psych exceptions are not derived from StandardError, so
Psych::SyntaxError will actually bypass almost any normal attempt to trap the
error, and will have wildly different details to the Syck errors.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>