avoid backquote in error messages, some uses in doc strings * libinterp/corefcn/quadcc.cc, libinterp/interpfcn/file-io.cc, libinterp/octave-value/ov-java.cc, scripts/image/colormap.m, scripts/image/rgbplot.m, scripts/miscellaneous/xor.m, scripts/plot/print.m, scripts/special-matrix/gallery.m, scripts/statistics/base/quantile.m, scripts/strings/strsplit.m: Use ' instead of ` in error messages. Don't use sprintf unnecessarily in error messages. Use @code or @samp instead of `' in doc strings.