vowpalwabbit/parse_regressor.cc: better error handling on -b mismatch

More information on mismatch, and a cleaner print, no need for exception when it is a simple user error.

Error used to look like:
```
$ ./vowpalwabbit/vw -b 18 -i model.19 test/train-sets/0001.dat
Wrong number of bits for source!
terminate called after throwing an instance of 'std::exception'
  what():  std::exception
Abort (core dumped)
```

Error now looks like this:
```
$ ./vowpalwabbit/vw -b 18 -i model.19 test/train-sets/0001.dat
vw: -b bits mismatch: command-line 18 != 19 stored in model

```