validator: Conform to the new parser and preprocessor output style

Because I overlooked the validator's dependency on the preprocessor
lineno_string() function, we wound up with badly-formatted output like
the following:

> 20140619 03:16:32 error validation:
> gui/default/widget/slider_minimal.cfg:53
>     included from gui/default.cfg:30
>     included from gui/default/widget/slider_minimal.cfg:100
>     included from gui/default.cfg:30: tag [wrong_tag] may not be used in [resolution]

This commit changes the validator's error formatting functions to
produce output more akin to the parser/preprocessor under the new
scheme introduced in version 1.11.10:

> 20140619 03:17:36 error validation: Tag [wrong_tag] may not be used in [resolution]
> at gui/default/widget/slider_minimal.cfg:53
>     included from gui/default.cfg:30
>     included from gui/default/widget/slider_minimal.cfg:100
>     included from gui/default.cfg:30

There are no translatable strings involved in this change, so this
commit may be safely backported to 1.12.

src/serialization/schema_validator.cpp

@@ -34,7 +34,7 @@ static lg::log_domain log_validation("validation");
 static std::string at(const std::string & file, int line){
 	std::ostringstream ss;
 	ss << line << " " << file;
-	return ::lineno_string(ss.str());
+	return "at " + ::lineno_string(ss.str());
 }
 
 static void print_output(const std::string & message,bool flag_exception = false ){
@@ -55,53 +55,59 @@ static void extra_tag_error(const std::string & file, int line,
 							const std::string & name,int n,
 							const std::string & parent, bool flag_exception){
 	std::ostringstream ss;
-	ss 	 <<at(file,line) << ": extra tag [" << name << "]; there may only be "
-			<< n << " ["<< name <<"] in [" << parent <<"]\n";
+	ss << "Extra tag [" << name << "]; there may only be "
+	   << n << " [" << name << "] in [" << parent << "]\n"
+	   << at(file, line) << "\n";
 	print_output (ss.str (),flag_exception);
 }
 
 static void wrong_tag_error(const std::string & file, int line,
 							const std::string & name,const std::string & parent,
 							bool flag_exception){
 	std::ostringstream ss;
-	ss 	 <<at(file,line) << ": tag [" << name << "] may not be used in [" <<
-			parent <<"]\n";
+	ss << "Tag [" << name << "] may not be used in ["
+	   << parent << "]\n"
+	   << at(file, line) << "\n";
 	print_output (ss.str (),flag_exception);
 }
 
 static void missing_tag_error(const std::string & file, int line,
 							  const std::string & name,int n,
 							  const std::string & parent, bool flag_exception){
 	std::ostringstream ss;
-	ss 	 <<at(file,line) << ": missing tag [" << name << "]; there must be "
-			<< n << " ["<<  name  <<"]s in [" << parent <<"]\n";
+	ss << "Missing tag [" << name << "]; there must be "
+	   << n << " [" <<  name  << "]s in [" << parent << "]\n"
+	   << at(file, line) << "\n";
 	print_output (ss.str (),flag_exception);
 }
 
 static void extra_key_error(const std::string & file, int line,
 					 const std::string & tag,const std::string & key,
 					 bool flag_exception){
 	std::ostringstream ss;
-	ss << at(file,line) << ": Invalid key '"<< key <<"=' in tag ["<< tag
-			<< "] on line " << line  << "\n";
+	ss << "Invalid key '" << key << "=' in tag [" << tag
+	   << "] on line " << line  << "\n"
+	   << at(file, line) << "\n";
 	print_output (ss.str (),flag_exception);
 }
 
 static void missing_key_error(const std::string & file, int line,
 					 const std::string & tag,const std::string & key,
 					 bool flag_exception){
 	std::ostringstream ss;
-	ss << at(file,line) << ": In tag "<< tag
-			<< " which begins here, " << " missing key "<< key << "\n";
+	ss << "In tag " << tag
+	   << " which begins here, " << " missing key " << key << "\n"
+	   << at(file, line) << "\n";
 	print_output (ss.str (),flag_exception);
 }
 
 static void wrong_value_error(const std::string & file, int line,
 					 const std::string & tag,const std::string & key,
 					 const std::string & value,bool flag_exception){
 	std::ostringstream ss;
-	ss << at(file,line) << ": Invalid value '"<< value << "' in key '" << key <<
-			"=' in tag ["<< tag <<"] on line " << line << "'\n";
+	ss << "Invalid value '" << value << "' in key '" << key
+	   << "=' in tag [" << tag << "] on line " << line << "'\n"
+	   << at(file, line) << "\n";
 	print_output (ss.str (),flag_exception);
 }