[TODO] Create an enum for the return values of the expander and use it for internal error reporting only

[itislu]

The same way as the heredoc_status enum.
This can replace the BAD_SUBSTITUTION exit code macro.

Originally posted by @itislu in #153 (comment)

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These are the exit codes defined in the bash source code:

/* Values that can be returned by execute_command (). */
#define EXECUTION_FAILURE 1
#define EXECUTION_SUCCESS 0

/* Usage messages by builtins result in a return status of 2. */
#define EX_BADUSAGE	2

#define EX_MISCERROR	2

/* Special exit statuses used by the shell, internally and externally. */
#define EX_RETRYFAIL	124
#define EX_WEXPCOMSUB	125
#define EX_BINARY_FILE	126
#define EX_NOEXEC	126
#define EX_NOINPUT	126
#define EX_NOTFOUND	127

#define EX_SHERRBASE	256	/* all special error values are > this. */

#define EX_BADSYNTAX	257	/* shell syntax error */
#define EX_USAGE	258	/* syntax error in usage */
#define EX_REDIRFAIL	259	/* redirection failed */
#define EX_BADASSIGN	260	/* variable assignment error */
#define EX_EXPFAIL	261	/* word expansion failed */
#define EX_DISKFALLBACK	262	/* fall back to disk command from builtin */

Originally posted by @itislu in #153 (comment)