reader: add a few more read errors and read_err_to_cstr

include/alisp/reader.h

@@ -8,13 +8,18 @@
 #ifndef READER_H
 #define READER_H
 
+#include <alisp/lisp.h>
 #include <alisp/stream.h>
 
 typedef enum
 {
-  READ_OK = 0,
+  READ_ERR_OK = 0,
+  READ_ERR_EOF,
+  READ_ERR_UNKNOWN_CHAR,
 } read_err_t;
 
+const char *read_err_to_cstr(read_err_t);
+
 // Attempt to read an expression from the stream, storing it in a pointer,
 // returning any errors if failed.
 read_err_t read(sys_t *, stream_t *, lisp_t **);