16.2 User-Supplied Error Handlers
  It is good programming practice to anticipate certain errors
  and provide your own error handlers for them. User-written
  error handlers allow you to handle errors for a specified
  block of program statements as well as complete program
  units. Any program module can contain one or more error
  handlers. These error handlers test the error condition and
  include statements to be executed if an error occurs.

  To provide your own error handlers, you use WHEN ERROR
  constructs. A WHEN ERROR construct consists of two blocks
  of code: a protected region and a handler. A protected re-
  gion is a block of code that is monitored by the compiler for
  the occurrence of an error. A handler is the block of code
  that receives program control when an error occurs during
  the execution of the statements in the protected region.

  There are two forms of WHEN ERROR constructs; in both
  cases the protected region begins immediately after a WHEN
  ERROR statement. The following partial programs illustrate
  each form. In Example 16-1, the handler is attached to the
  protected region, while in Example 16-2, the handler catch_
  handler is detached and must be provided elsewhere in the
  program unit.

  The following sections further explain the concepts of pro-
  tected regions and handlers.