Add call context when collecting errors.

Author: gkellogg

lib/rdf/reader.rb

@@ -540,7 +540,7 @@ def lineno
     def valid?
       super && !log_statistics[:error]
     rescue ArgumentError, RDF::ReaderError => e
-      log_error(e.message)
+      log_error(e.message + " at #{e.backtrace.first}")
       false
     end
 

lib/rdf/writer.rb

@@ -509,7 +509,7 @@ def write_statement(statement)
       end
       self
     rescue ArgumentError => e
-      log_error e.message
+      log_error e.message + " at #{e.backtrace.first}"
     end
     alias_method :insert_statement, :write_statement # support the RDF::Writable interface