The basic principle of shaders is the preference of replacement. By that I mean they will replace a basic texture of the same name. This works as well for the textures of models or effects. This means that if your shader entry is named exactly the same as a physical texture in the textures folder, the shader will replace the basic texture, and the basic texture won't exist anymore, in a sense.
If the shader entry is a novel name, then it will be effectively a new texture, and it requires an editorimage attribute in the shader code for it to properly be shown in Radiant, as a physical texture by that name doesn't exist. However, the editorimage won't be necessary for the game itself. Furthermore, nothing prevents from giving a physical texture name entry (replacement shader) a "conflicting" editorimage, although it's not necessary, but sometimes convenient.
If you fail to provide an editorimage for a novel shader entry name, or enter a nonexisting editorimage, Radiant won't be able to locate any image for the shader to be displayed in the editor. This will result in that shader not found error texture. For example some Raven editorimages are missing, I think.
Frozen by ICARUS