(puppetlabs#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