PR# 17568 Prettifyer needs more specific error messages

I tried to prettify a class via

   ec -pretty test.e

The output was

   Prettifyer: Unable to prettify.

After a few minutes of head scratching, I realized that test.e was in
a sub-directory (it was in classes/test.e), so the file I named on the
command line did not exist.

The prettifyer should give more specific error messages about what is wrong,
if possible.  In particular, a nonexistent file, a file with permission 
problems or a "file" that is really a directory ought to elicit a more useful
error message.

If the "file" I try to prettify is a really directory and is NFS-mounted 
instead of on a local file system, ec crashes when I try to prettify it:

marten 316% ec -pretty /home/dhollen
ISE Eiffel: Session aborted
Exception tag: FILE: unable to read stream.

ec: system execution failed.
Following is the set of recorded exceptions:

******************************** Thread exception *****************************
In thread           Root thread            0x0 (thread id)
*******************************************************************************
-------------------------------------------------------------------------------
Class / Object      Routine                Nature of exception           Effect
-------------------------------------------------------------------------------
KL_BINARY_INPUT_FILE
                    read_to_string @1      FILE: unable to read stream.:
<00000000062F6970>  (From RAW_FILE)        Runtime I/O error.            Fail
-----------------------------------------
....
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