Avoid interop build instructions when already built

This reduces the amount of logspam generated during interop testing.
Telling someone how to build when they have already obviously built
doesn't serve much purpose, now that everyone is used to it not
building automatically.

run-test-client.sh

@@ -1,9 +1,11 @@
 #!/bin/bash -e
 cd "$(dirname "$0")"
-cat >&2 <<EOF
-Gradle is no longer run automatically. Make sure to run
-'./gradlew installDist -PskipCodegen=true' or
-'./gradlew :grpc-interop-testing:installDist -PskipCodegen=true' after any
-changes. -PskipCodegen=true is optional, but requires less setup.
+BIN="./interop-testing/build/install/grpc-interop-testing/bin/test-client"
+if [[ ! -e "$BIN" ]]; then
+  cat >&2 <<EOF
+Could not find binary. It can be built with:
+./gradlew :grpc-interop-testing:installDist -PskipCodegen=true
 EOF
-exec ./interop-testing/build/install/grpc-interop-testing/bin/test-client "$@"
+  exit 1
+fi
+exec "$BIN" "$@"

run-test-server.sh

@@ -1,9 +1,11 @@
 #!/bin/bash -e
 cd "$(dirname "$0")"
-cat >&2 <<EOF
-Gradle is no longer run automatically. Make sure to run
-'./gradlew installDist -PskipCodegen=true' or
-'./gradlew :grpc-interop-testing:installDist -PskipCodegen=true' after any
-changes. -PskipCodegen=true is optional, but requires less setup.
+BIN="./interop-testing/build/install/grpc-interop-testing/bin/test-server"
+if [[ ! -e "$BIN" ]]; then
+  cat >&2 <<EOF
+Could not find binary. It can be built with:
+./gradlew :grpc-interop-testing:installDist -PskipCodegen=true
 EOF
-exec ./interop-testing/build/install/grpc-interop-testing/bin/test-server "$@"
+  exit 1
+fi
+exec "$BIN" "$@"