Validate uploaded PDB files in browser

[sverhoeven]

Is your feature request related to a problem? Please describe.
Currently you can upload a PDB file and the only check is that it has a .pdb. extension.
If an invalid PDB file is uploaded the 3D viewer has an empty canvas and no chains or residues can be selected.

Describe the solution you'd like
It would be nice to run some validation and give a nice error when invalid.
This would need a pdb parser like the one shipped in NGL.
Can not use https://github.com/i-VRESSE/pdbtbx-ts as it is to strict.