Requires that input is a char row vector.
Raises an error if the input x is not a row vector of
char row vectors are Octave’s normal representation of single strings.
(They are what are produced by
'...' string literals.) As a special
case, 0-by-0 empty chars (what is produced by the string literal
are also considered charvecs.
This does not differentiate between single-quoted and double-quoted strings.
label is an optional input that determines how the input will be described in
error messages. If not supplied,
inputname (1) is used, and if that is
empty, it falls back to "input".