Improve handling of errors

[martimunicoy]

Description

Some errors of Frag are still s mystery. At least, we need to be able to describe common scenarios:

• When the first growing step cannot accept any pose. This could mean that the initial miniaturised pose does not fit well to the cavity. A possible message could be: Initial fragment could not be accepted by PELE. It looks like this fragment does not fit well to the cavity. You might have to try another protein conformation or set a higher number of PELE steps.
• When the linking atoms are not defined correctly, for example some of them are missing in the structure.