Change to fail the cache addInstaller if it already exists. (#40)

Author: jshum2479

imagetool/src/main/java/com/oracle/weblogic/imagetool/cli/cache/AddInstallerEntry.java

@@ -31,6 +31,12 @@ public AddInstallerEntry(boolean isCLIMode) {
     public CommandResponse call() {
         if (location != null && Files.isRegularFile(location) && !Utils.isEmptyString(version)) {
             String key = String.format("%s%s%s", type, CacheStore.CACHE_KEY_SEPARATOR, version);
+            if (cacheStore.getValueFromCache(key) != null) {
+                return new CommandResponse(-1, String.format("Installer already exists %s=%s. Try removing it "
+                        + "using the deleteEntry command before adding it again.",
+                    key,
+                    cacheStore.getValueFromCache(key)));
+            }
             cacheStore.addToCache(key, location.toAbsolutePath().toString());
             return new CommandResponse(0, String.format("Successfully added to cache. %s=%s", key,
                     cacheStore.getValueFromCache(key)));