confusing set_all_manifold* errors if empty
confusing set_all_manifold* errors if empty

The error messages for set_all_manifold_ids* are confusing if the mesh
is empty:

An error occurred in line <10760> of file </ssd/deal-
git/source/grid/tria.cc> in function
    dealii::Triangulation<dim, spacedim>::raw_quad_iterator
dealii::Triangulation<dim, spacedim>::begin_raw_quad(unsigned int) const
[with int dim = 2, int spacedim = 2, dealii::Triangulation<dim,
spacedim>::raw_quad_iterator =
dealii::TriaRawIterator<dealii::CellAccessor<2, 2> >]
The violated condition was:
    level<n_global_levels() || level<levels.size()
The name and call sequence of the exception was:
    ExcInvalidLevel(level)
Additional Information:
The given level 0 is not in the valid range!

Fix that.

EGP-CIG-REU/dealii,angelrca/dealii,kalj/dealii,naliboff/dealii,JaeryunYim/dealii,jperryhouts/dealii,andreamola/dealii,maieneuro/dealii,andreamola/dealii,angelrca/dealii,pesser/dealii,naliboff/dealii,danshapero/dealii,gpitton/dealii,nicolacavallini/dealii,jperryhouts/dealii,sairajat/dealii,gpitton/dealii,naliboff/dealii,danshapero/dealii,kalj/dealii,andreamola/dealii,sairajat/dealii,nicolacavallini/dealii,shakirbsm/dealii,sairajat/dealii,ESeNonFossiIo/dealii,kalj/dealii,gpitton/dealii,gpitton/dealii,andreamola/dealii,ESeNonFossiIo/dealii,nicolacavallini/dealii,andreamola/dealii,EGP-CIG-REU/dealii,JaeryunYim/dealii,maieneuro/dealii,sairajat/dealii,maieneuro/dealii,shakirbsm/dealii,andreamola/dealii,jperryhouts/dealii,spco/dealii,jperryhouts/dealii,danshapero/dealii,nicolacavallini/dealii,angelrca/dealii,pesser/dealii,angelrca/dealii,pesser/dealii,pesser/dealii,spco/dealii,EGP-CIG-REU/dealii,jperryhouts/dealii,kalj/dealii,angelrca/dealii,spco/dealii,danshapero/dealii,angelrca/dealii,maieneuro/dealii,ESeNonFossiIo/dealii,shakirbsm/dealii,danshapero/dealii,ESeNonFossiIo/dealii,pesser/dealii,gpitton/dealii,ESeNonFossiIo/dealii,angelrca/dealii,pesser/dealii,spco/dealii,nicolacavallini/dealii,EGP-CIG-REU/dealii,sairajat/dealii,spco/dealii,jperryhouts/dealii,shakirbsm/dealii,naliboff/dealii,kalj/dealii,naliboff/dealii,nicolacavallini/dealii,kalj/dealii,shakirbsm/dealii,maieneuro/dealii,JaeryunYim/dealii,ESeNonFossiIo/dealii,JaeryunYim/dealii,nicolacavallini/dealii,spco/dealii,danshapero/dealii,maieneuro/dealii,danshapero/dealii,JaeryunYim/dealii,EGP-CIG-REU/dealii,gpitton/dealii,spco/dealii,jperryhouts/dealii,naliboff/dealii,naliboff/dealii,JaeryunYim/dealii,pesser/dealii,kalj/dealii,ESeNonFossiIo/dealii,EGP-CIG-REU/dealii,maieneuro/dealii,EGP-CIG-REU/dealii,andreamola/dealii,sairajat/dealii,JaeryunYim/dealii,sairajat/dealii,gpitton/dealii,shakirbsm/dealii,shakirbsm/dealii