provide direct feedback to user in case of problematic pdb file

[bvreede]

In #69 the pdbtbx-ts library is used to parse pdb files provided as input by the user. This library can be strict, which leads to the builder not being able to provide chains and residue numbers cf. the input file, if there is a problem in this file.

It would be good to have a test early on, when a file is uploaded, that confirms that the file can be adequately parsed; if not, provide feedback to the user at this point.