Add concept of error messages.

include/error.h

@@ -0,0 +1,30 @@
+#ifndef _BOOTSTRAP_ERROR_H
+#define _BOOTSTRAP_ERROR_H
+
+enum ErrorCode {
+  SUCCESS = 0,
+
+  ERROR_CONFIG_ENV_CWD_INVALID,
+  ERROR_CONFIG_ENV_ROOT_DIR_INVALID,
+  ERROR_CONFIG_TARGET_NOT_FOUND,
+  ERROR_CONFIG_TARGET_NOT_DIR,
+
+  ERROR_PARSER_SPEC_JSON_CANNOT_OPEN,
+  ERROR_PARSER_SPEC_JSON_INVALID_SYNTAX,
+
+  ERROR_VALIDATOR_TOP_LEVEL_NOT_OBJECT,
+  ERROR_VALIDATOR_FIELD_NOT_OBJECT,
+  ERROR_VALIDATOR_FIELD_TYPE_INVALID,
+  ERROR_VALIDATOR_FIELD_TYPE_UNKNOWN,
+  ERROR_VALIDATOR_FIELD_PROMPT_INVALID,
+
+  ERROR_EVALUATOR_RUN_SH_NOT_FOUND,
+  ERROR_EVALUATOR_RESPONSE_INVALID,
+};
+
+struct Error {
+  enum ErrorCode code;
+  const char *message;
+};
+
+#endif /* _BOOTSTRAP_ERROR_H */