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.

TCL_OK                   0

TCL_ERROR              1

TCL_RETURN            2

TCL_BREAK              3

TCL_CONTINUE         4