Only the Tcl keyword commands (Built-in commands) return an error. Most ACTION commands do not. When a TCL command returns, the interpreter contains a result from the command. These returns can be modified by user-defined messages (using the return command).
TCL_OK Command completed normally; the interpreter's result contains the command's result.
TCL_ERROR The command couldn't be completed successfully the interpreter's result describes what went wrong.
TCL_RETURN The command requests that the current procedure return the interpreter's result contains the procedure's return value.
TCL_BREAK The command requests that the innermost loop be exited; the interpreter's result is meaningless.
TCL_CONTINUE Go on to the next iteration of the current loop the interpreter's result is meaningless.