file_key_management plugin is not installable at runtime

I don't see how to install file_key_management plugin at runtime.
The plugin init requires file_key_management_filename variable to be set, and to set the variable one needs the plugin.
The part about file_key_management_filename being mandatory is documented, so I assume the whole thing is intentional. However, it would be nice to produce a better error message. Now it looks like this:

MariaDB [test]> install soname 'file_key_management';

ERROR 29 (HY000): File '' not found (Errcode: 2 "No such file or directory")

and the error log says

150411 19:28:06 [ERROR] Plugin 'file_key_management' init function returned error.

150411 19:28:06 [ERROR] Plugin 'file_key_management' registration as a ENCRYPTION failed.

Both are not very helpful.
Probably the best would be to say that the plugin cannot be installed at runtime, but I don't know if it's possible.
Also, if it's not installable at runtime, it should be documented.