Produce a more informative error when someone asks RDKit to load mol2

[j-wags]

Is your feature request related to a problem? Please describe.
_From @dfhahn _
Another issue I've come across as a newbie @jeffrey Wagner
I want to import a mol2 file without having openeye installed. This is not supported as we heard in your demo talk. However, the error message should mention this. Otherwise people think it is an error and might start doing stuff like I did (and which is not healthy I guess):

from rdkit.Chem import AllChem
from simtk.openmm.app import PDBFile
from openforcefield.topology import Molecule, Topology
from openforcefield.typing.engines.smirnoff import ForceField

romol = AllChem.MolFromMol2File(ligand_path)
ligand = Molecule.from_rdkit(romol)

Describe the solution you'd like
We could catch the specific text of this error in Molecule.from_file, and add to the error message

[j-wags]

#425 adds substantially to the error message.
NotImplementedError: No toolkits in registry can read file ligand1.mol2 (format MOL2). Supported formats in the provided ToolkitRegistry are {'The RDKit': ['SDF', 'MOL', 'SMI'], 'AmberTools': []}. RDKit does not fully support input of molecules from mol2 format unless they have Corina atom types, and this is not common in the simulation community. For this reason, the Open Force Field Toolkit does not use RDKit to read .mol2. Consider reading from SDF instead. If you would like to attempt to use RDKit to read mol2 anyway, you can load the molecule of interest into an RDKit molecule and use openforcefield.topology.Molecule.from_rdkit, but we do not recommend this.