add SB-EXT:*SUPPRESS-PRINT-ERRORS* modelled after *BREAK-ON-SIGNALS*

add SB-EXT:*SUPPRESS-PRINT-ERRORS* modelled after *BREAK-ON-SIGNALS*

  When non-NIL, OUTPUT-OBJECT (our main entry to the printer) binds a handler
  that handles conditions of the specified type by printing an error marker
  instead of signaling an error.

  WRITE also accepts :SUPPRESS-ERRORS, and WITH-STANDARD-IO-SYNTAX binds it to
  NIL.

  Calls SIGNAL before handling the condition so outer handlers get a chance to
  use restarts, etc.