Unclear bug message

[aguschin]

From #397 (comment)

Unclear bug message in release/0.3.0:

$ mlem declare env docker --registry local --daemon.host localhost docker-builder.mlem                         124ms
💾 Saving env to docker-builder.mlem
$ mlem build docker --file_conf docker-builder --model rf.mlem --image.name=rf-docker                         6.809s
❌ Unexpected error: not enough values to unpack (expected 2, got 1)
Please report it here: <https://github.com/iterative/mlem/issues>

Should be something like

File provided to `file_conf` option must have lines of form "arg=value"

[aguschin]

Btw, this should not be --file_conf. It should be --load. Let's fix that as well.