Small script fix (#2591)

# Objective
Prevent some possible problem
Found by IntelliJ IDEA

## Solution

Double quoting variable and exit on possible failure of cd command.

tools/example_showcase.sh

@@ -3,16 +3,16 @@
 duration='2'
 function wait_seconds() { perl -e 'alarm shift; exec @ARGV' "$@"; }
 run_example() {
-    cargo build --example $1
-    wait_seconds "$duration" cargo run --example $1
+    cargo build --example "$1"
+    wait_seconds "$duration" cargo run --example "$1"
 }
 
 example_list="$(cargo build --example 2>&1)"
 example_list=${example_list//$'\n'/}
 example_list="${example_list#error\: \"--example\" takes one argument.Available examples\: }"
-echo $example_list
+echo "$example_list"
 for example in $example_list
 do
     echo "Running example: $example"
-    run_example $example
+    run_example "$example"
 done

tools/publish.sh

@@ -37,7 +37,7 @@ cd crates
 for crate in "${crates[@]}"
 do
   echo "Publishing ${crate}"
-  (cd $crate; cargo publish --no-verify)
+  (cd "$crate"; cargo publish --no-verify)
   sleep 20
 done