Execute a slave command, notifying user and checking for errors.
Output from COMMAND goes to BUFFER, or the current buffer if
BUFFER is t. If the destination buffer is not already current,
set it up properly and erase it. The command is considered
successful if its exit status does not exceed OKSTATUS (if
OKSTATUS is nil, that means to ignore error status, if it is
`async', that means not to wait for termination of the
subprocess; if it is t it means to ignore all execution errors).
FILE-OR-LIST is the name of a working file; it may be a list of
files or be nil (to execute commands that don't expect a file
name or set of files). If an optional list of FLAGS is present,
that is inserted into the command line before the filename.
Return the return value of the slave command in the synchronous
case, and the process object in the asynchronous case.
(fn BUFFER OKSTATUS COMMAND FILE-OR-LIST &rest FLAGS)