add SystemErrno to Error (#740)

* adding SystemErrno to Error, and fixing error logic when open fails

* fix for old versions of libsqlite3 that do not have sqlite3_system_errno defined

* fixing pre-processor logic

error.go

@@ -5,7 +5,15 @@
 
 package sqlite3
 
+/*
+#ifndef USE_LIBSQLITE3
+#include <sqlite3-binding.h>
+#else
+#include <sqlite3.h>
+#endif
+*/
 import "C"
+import "syscall"
 
 // ErrNo inherit errno.
 type ErrNo int
@@ -20,6 +28,7 @@ type ErrNoExtended int
 type Error struct {
 	Code         ErrNo         /* The error code returned by SQLite */
 	ExtendedCode ErrNoExtended /* The extended error code returned by SQLite */
+	SystemErrno  syscall.Errno /* The system errno returned by the OS through SQLite, if applicable */
 	err          string        /* The error string returned by sqlite3_errmsg(),
 	this usually contains more specific details. */
 }
@@ -72,10 +81,16 @@ func (err ErrNoExtended) Error() string {
 }
 
 func (err Error) Error() string {
+	var str string
 	if err.err != "" {
-		return err.err
+		str = err.err
+	} else {
+		str = C.GoString(C.sqlite3_errstr(C.int(err.Code)))
 	}
-	return errorString(err)
+	if err.SystemErrno != 0 {
+		str += ": " + err.SystemErrno.Error()
+	}
+	return str
 }
 
 // result codes from http://www.sqlite.org/c3ref/c_abort_rollback.html