properly message errors stemming from truncated parameter files

[MarioStanke]

When a training process does not properly finish (e.g. multiple training processes write to the same species' directory, an interruption), augustus can come up with cryptic error messages.

An example to reproduce such an error:

cd Augustus/config/species/fly
rm fly_exon_probs.pbl
touch fly_exon_probs.pb

augustus --species=fly examples/example.fa

produces

augustus: ERROR
        Invalid nucleotide '�' encountered.

There should be an error message that reports a problem in the specific file. It is also possible, although I have not tested this, that different partial or corrupt parameter files lead to a segmentation fault.