which => command -v; different format.sh exit codes

`which` is a debian tool that is being deprecated in favour of
posix-standard `command -v`, so which to that to avoid deprecation
warnings.

Change the exit codes of `contrib/format.sh verify` to be unique for
each formatting program.

contrib/format.sh

@@ -2,12 +2,12 @@
 
 CLANG_FORMAT_DESIRED_VERSION=11
 
-binary=$(which clang-format-$CLANG_FORMAT_DESIRED_VERSION 2>/dev/null)
+binary=$(command -v clang-format-$CLANG_FORMAT_DESIRED_VERSION 2>/dev/null)
 if [ $? -ne 0 ]; then
-    binary=$(which clang-format-mp-$CLANG_FORMAT_DESIRED_VERSION 2>/dev/null)
+    binary=$(command -v clang-format-mp-$CLANG_FORMAT_DESIRED_VERSION 2>/dev/null)
 fi
 if [ $? -ne 0 ]; then
-    binary=$(which clang-format 2>/dev/null)
+    binary=$(command -v clang-format 2>/dev/null)
     if [ $? -ne 0 ]; then
         echo "Please install clang-format version $CLANG_FORMAT_DESIRED_VERSION and re-run this script."
         exit 1
@@ -22,18 +22,18 @@ fi
 cd "$(dirname $0)/../"
 if [ "$1" = "verify" ] ; then
     if [ $($binary --output-replacements-xml $(find jni daemon llarp include pybind | grep -E '\.([hc](pp)?|mm?)$' | grep -v '\#') | grep '</replacement>' | wc -l) -ne 0 ] ; then
-        exit 1
+        exit 2
     fi
 else
     $binary -i $(find jni daemon llarp include pybind | grep -E '\.([hc](pp)?|mm)$' | grep -v '\#') &> /dev/null
 fi
 
-swift_format=$(which swiftformat 2>/dev/null)
+swift_format=$(command -v swiftformat 2>/dev/null)
 if [ $? -eq 0 ]; then
     if [ "$1" = "verify" ] ; then
         for f in $(find daemon | grep -E '\.swift$' | grep -v '\#') ; do
             if [ $($swift_format --quiet --dryrun < "$f" | diff "$f" - | wc -l) -ne 0 ] ; then
-                exit 1
+                exit 3
             fi
         done
     else