bug#10082: 23.3; many Rmail commands fail to do the right thing on an empty inbox [MINOR]

[Mark Lillibridge]

    Visit an empty Rmail inbox.  Hit '.' and notice a reasonable "No
mail." error message.  This is reasonable behavior.  A few of the other
Rmail commands also produce reasonable error messages; for example, M-m
produces "Cannot parse this as a failure message".

    However, the following commands do not produce a reasonable error
message and fail after asking for input with weird error messages like 
Search failed: "

":

f, o, C-o, k, a, w

A check for no message should probably be added to these commands.

- Mark