plugin -remove deletes bin directory

I tried to install a plugin directly from disk, but used the wrong arguments. Totally my fault for not RTFM properly.

However, by following the instructions printed out by `bin/plugin`, I ended up deleting my $ES/bin directory.

```
$ pwd
/tmp/elasticsearch-0.90.5
$ ls
LICENSE.txt    NOTICE.txt     README.textile bin            config         lib
$ bin/plugin --install file:///tmp/foo.zip
-> Installing file:///tmp/foo.zip...
Failed to install file:///tmp/foo.zip, reason: plugin directory /tmp/elasticsearch-0.90.5/plugins already exists. To update the plugin, uninstall it first using -remove file:///tmp/foo.zip command
$ bin/plugin -remove file:///tmp/foo.zip
-> Removing file:///tmp/foo.zip
Removed file:///tmp/foo.zip
$ ls
LICENSE.txt    NOTICE.txt     README.textile config         lib
```

I reproduced the problem in 0.90.5 and the latest master.
Closes #3847.

src/main/java/org/elasticsearch/plugins/PluginManager.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.plugins;
 
+import com.google.common.base.Strings;
+import org.elasticsearch.ElasticSearchIllegalArgumentException;
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.http.client.HttpDownloadHelper;
@@ -221,6 +223,10 @@ public class PluginManager {
         PluginHandle pluginHandle = PluginHandle.parse(name);
         boolean removed = false;
 
+        if (Strings.isNullOrEmpty(pluginHandle.name)) {
+            throw new ElasticSearchIllegalArgumentException("plugin name is incorrect");
+        }
+
         File pluginToDelete = pluginHandle.extractedDir(environment);
         if (pluginToDelete.exists()) {
             debug("Removing: " + pluginToDelete.getPath());
@@ -331,6 +337,7 @@ public class PluginManager {
                         // Deprecated commands
                         || command.equals("remove") || command.equals("-remove")) {
                     pluginName = args[++c];
+
                     action = ACTION.REMOVE;
                 } else if (command.equals("-l") || command.equals("--list")) {
                     action = ACTION.LIST;
@@ -371,6 +378,9 @@ public class PluginManager {
                         pluginManager.log("-> Removing " + pluginName + " ");
                         pluginManager.removePlugin(pluginName);
                         exitCode = EXIT_CODE_OK;
+                    } catch (ElasticSearchIllegalArgumentException e) {
+                        exitCode = EXIT_CODE_CMD_USAGE;
+                        pluginManager.log("Failed to remove " + pluginName + ", reason: " + e.getMessage());
                     } catch (IOException e) {
                         exitCode = EXIT_CODE_IO_ERROR;
                         pluginManager.log("Failed to remove " + pluginName + ", reason: " + e.getMessage());