Exception handler: Unknown error

Very often "Unknown error" is shown and the exception that has caused the last exception is not shown. The message from the exception handler should include the causing exception(s).

Example

Unknown error: /pfs/data5/home/kit/scc/jk7683/vre-language/examples/atoms.vm:3:9 --> Structure from (
         (atoms: ((symbols: 'H', 'H', 'O'),
                  (x: 1., 2., 3.) [nm],
                  (y: 4., 5., 6.) [angstrom],
                  (z: 7., 8., 9.) [angstrom],
                  (tags: 'molec', 'mol', 'molecule')
                 )
         ),
         (cell: [[1., 2, 3], [1, 2, 3], [1, 2, 3]] [bohr]),
         (pbc: [false, false, true]),
         (charge: -2.e-19) [C]
       ) <--
Expected a 1D array, got an array with shape (3, 3)

The causing exception is a KeyError and it is not shown. Therefore the message is confusing.