(#1532) Replace ParserError with Puppet::Error

I'm not sure how we ended up with ParserError in the provider. This
exception doesn't exist in Ruby. Puppet ships ther own exception,
Puppet::Error. It probably makes sense to raise that instead.

lib/puppet/provider/postgresql_conf/ruby.rb

@@ -75,7 +75,7 @@ def write_config(file, lines)
   # check, if resource exists in postgresql.conf file
   def exists?
     select = parse_config.select { |hash| hash[:key] == resource[:key] }
-    raise ParserError, "found multiple config items of #{resource[:key]} found, please fix this" if select.length > 1
+    raise Puppet::Error, "found multiple config items of #{resource[:key]} found, please fix this" if select.length > 1
     return false if select.empty?
 
     @result = select.first

spec/unit/provider/postgresql_conf/ruby_spec.rb

@@ -30,6 +30,13 @@
     expect(provider).to respond_to(:exists?)
   end
 
+  describe 'duplicate keys in postgresql.conf' do
+    it 'raises an exception' do
+      allow(provider).to receive(:parse_config).and_return([{key: 'foo', line: 1}, {key: 'foo', line: 2}])
+      expect(provider.exists?).to raise_error(Puppet::Error, 'found multiple config items of foo found, please fix this')
+    end
+  end
+
   it 'has a method create' do
     expect(provider).to respond_to(:create)
   end