Can't open Eclipse after installing ADT Plugin

I just followed the steps on http://developer.android.com/sdk/installing/installing-adt.html#Download to download and install the ADT plugin for Eclipse on my Mac. As soon as it finished downloading, I tried opening Eclipse to finish configuring the ADT. I, however, cannot open Eclipse anymore. All I get is the following error:

How can I fix this error and get Eclipse to open again? What are the correct steps to successfully install the ADT plugin?

I was able to get Eclipse to open again without a complete reinstallation, remove all the bad ADT files, and then successfully install the ADT plugin with the following steps:

^{(I don't know if this method is correct, but it worked)}

Get Eclipse to open:

1. Download a new copy of Eclipse into a different folder.
2. From the new Eclipse folder, copy only the Eclipse.app file into the old Eclipse root directory.

I was then able to open Eclipse by clicking on the new Eclipse.app in my old Eclipse folder. All my old projects and preferences were still there.

Delete Bad ADT Files:

1. In Eclipse Help > Installation Details > select all Android files and click uninstall
2. In Eclipse Help > Eclipse Marketplace > search for adt > uninstall the Android Development Tools for Eclipse

Reinstall the ADT Plugin:

I then had problems reinstalling the ADT Plugin and was getting the following error message:

An error occurred during the org.eclipse.equinox.internal.p2.engine.phases.CheckTrust phase...

Following the directions I found here, I deleted the artifacts.xml file in the Eclipse root directory. I then relaunched Eclipse and was able to install the ADT Plugin through Help > Eclipse Marketplace > search for ADT > click the install button on the ADT package > relaunch Eclipse.

Update Note:

As mentioned in the Android developer documentation, since June 2015, the Eclipse ADT plugin is no longer supported and it is recommended to switch to Android Studio.