File access error handling

The change object file access popup has an attref called, "Error Result"

If it is left at its default value of "Null" (the Null system attribute)
then a file access error will stop the run, as current. If it is pointed
at an attribute, then instead of the model stopping, the attribute is set
to the following value after the file operation completes:

-1 : user cancelled selecting a file (with browse before load/save option on) or chose not to overwrite an existing file
0 : No Error
1 : Error opening file
2 : EOF occured while reading the file (not signalled for reading a variable length table 4.29k)
3 : Error renaming a written temporary file to its proper name
4 : Incomplete row in table data
5 : Data contains unknown label
6 : Data does not match expected value format
7 : Data did not decode as a plain number
8 : FATAL error - platform will terminate run soon
9 : Unknown error reason (catch all)

More error classes may be added in the future.

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idkbase note 59