Improve module's abstract

Ref: http://blogs.perl.org/users/neilb/2014/07/give-your-modules-a-good-abstract.html
Simplify error reporting

The Gerrit::REST::Exception was an attempt to provide strutured
exceptions for this module. But its implementation and usage was
becoming more complex than useful. So, we get rid of it and change the
interface so that now error messages are consistently formatted and
thrown using Carp::croak.
Improve documentation
Placate Perl::Critic
Update README.pod
v0.010

  [Changes]

  - The Gerrit::REST::Exception module was an attempt to provide structured
  exceptions for this module. But its implementation and usage was becoming
  more complex than useful. So, we get rid of it and change the interface so
  that now error messages are consistently formatted and thrown using
  Carp::croak.