(FM-8643) Ensure library requires fail usefully

This commit places the requires statement for the
ruby-pwsh ruby code (found in the dependency module,
puppetlabs-pwshlib) in a begin-rescue block and adds
a loud error message that points at the solution.

This will cause the loading of the providers to still
fail during autoloading, but to do so usefully.

We chose not to use the feature/confine pattern
because it is not supported by the resource_api, the
primary interface through which we expect community
members to write types and providers in the future.

We chose not to use a utility loader method, which
would have prevented breaking during autoloading,
because of the friction and additional complexity
cost incurred for every module needing to adopt
this pattern.

While this rescued-requires pattern will still
cause an error during autoloading, this should
only be possible in cases where the module is
being installed from github and the dependency
chain is not installed.

Author: michaeltlombardi

.rubocop_todo.yml

@@ -1,19 +1,20 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2019-10-18 11:01:56 -0500 using RuboCop version 0.49.1.
+# on 2019-10-30 10:31:01 -0500 using RuboCop version 0.49.1.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
 # versions of RuboCop, may require this file to be generated again.
 
-# Offense count: 4
+# Offense count: 6
 GetText/DecorateFunctionMessage:
   Exclude:
     - 'lib/puppet/provider/exec/powershell.rb'
     - 'lib/puppet/provider/exec/pwsh.rb'
     - 'spec/spec_helper_acceptance.rb'
 
-# Offense count: 1
+# Offense count: 3
 GetText/DecorateString:
   Exclude:
     - 'lib/puppet/provider/exec/powershell.rb'
+    - 'lib/puppet/provider/exec/pwsh.rb'

lib/puppet/provider/exec/powershell.rb

@@ -1,5 +1,9 @@
 require 'puppet/provider/exec'
-require 'ruby-pwsh'
+begin
+  require 'ruby-pwsh'
+rescue LoadError
+  raise 'Could not load the "ruby-pwsh" library; is the dependency module puppetlabs-pwshlib installed in this environment?'
+end
 
 Puppet::Type.type(:exec).provide :powershell, :parent => Puppet::Provider::Exec do
   confine :operatingsystem => :windows

lib/puppet/provider/exec/pwsh.rb

@@ -1,5 +1,9 @@
 require 'puppet/provider/exec'
-require 'ruby-pwsh'
+begin
+  require 'ruby-pwsh'
+rescue LoadError
+  raise 'Could not load the "ruby-pwsh" library; is the dependency module puppetlabs-pwshlib installed in this environment?'
+end
 
 Puppet::Type.type(:exec).provide :pwsh, :parent => Puppet::Provider::Exec do
   desc <<-EOT