(maint) better error reporting for prefix and suffix

When prefix and suffix did error checking with positional arguments,
they would not report the position of the argument that failed to
validate. This commit changes the messages to indicate which argument
failed.

lib/puppet/parser/functions/prefix.rb

@@ -21,14 +21,14 @@ Will return: ['pa','pb','pc']
     array = arguments[0]
 
     unless array.is_a?(Array)
-      raise(Puppet::ParseError, 'prefix(): Requires array to work with')
+      raise Puppet::ParseError, "prefix(): expected first argument to be an Array, got #{array.inspect}"
     end
 
     prefix = arguments[1] if arguments[1]
 
     if prefix
       unless prefix.is_a?(String)
-        raise(Puppet::ParseError, 'prefix(): Requires string to work with')
+        raise Puppet::ParseError, "prefix(): expected second argument to be a String, got #{suffix.inspect}"
       end
     end
 

lib/puppet/parser/functions/suffix.rb

@@ -21,14 +21,14 @@ Will return: ['ap','bp','cp']
     array = arguments[0]
 
     unless array.is_a?(Array)
-      raise(Puppet::ParseError, 'suffix(): Requires array to work with')
+      raise Puppet::ParseError, "suffix(): expected first argument to be an Array, got #{array.inspect}"
     end
 
     suffix = arguments[1] if arguments[1]
 
     if suffix
-      unless suffix.is_a?(String)
-        raise(Puppet::ParseError, 'suffix(): Requires string to work with')
+      unless suffix.is_a? String
+        raise Puppet::ParseError, "suffix(): expected second argument to be a String, got #{suffix.inspect}"
       end
     end