chore: Improved messaging when yarn is not present. (aws#4899)

Currently, the message generated is 'lerna ERR! yarn run build exited 2'
which is not very helpful.

build.sh

@@ -51,6 +51,11 @@ rm -rf $BUILD_INDICATOR
 export MERKLE_BUILD_CACHE=$(mktemp -d)
 trap "rm -rf $MERKLE_BUILD_CACHE" EXIT
 
+if ! [ -x "$(command -v yarn)" ]; then
+  echo "yarn is not installed. Install it from here- https://yarnpkg.com/en/docs/install."
+  exit 1
+fi
+
 echo "============================================================================================="
 echo "building..."
 time lerna run $bail --stream $runtarget || fail

scripts/builddown

@@ -1,6 +1,11 @@
 #!/bin/bash
 set -euo pipefail
 
+if ! [ -x "$(command -v yarn)" ]; then
+  echo "yarn is not installed. Follow the guide to install yarn - https://yarnpkg.com/en/docs/install."
+  exit 1
+fi
+
 export NODE_OPTIONS="--max-old-space-size=4096 ${NODE_OPTIONS:-}"
 
 scriptdir=$(cd $(dirname $0) && pwd)

scripts/buildup

@@ -1,6 +1,11 @@
 #!/bin/bash
 set -euo pipefail
 
+if ! [ -x "$(command -v yarn)" ]; then
+  echo "yarn is not installed. Install it from here- https://yarnpkg.com/en/docs/install."
+  exit 1
+fi
+
 export NODE_OPTIONS="--max-old-space-size=4096 ${NODE_OPTIONS:-}"
 
 scriptdir=$(cd $(dirname $0) && pwd)