Reporting steps limit exceeded is awkward

[claudemarche]

Currently, when Alt-Ergo reaches the steps limit given using --steps-bound, the reporting is weird, e.g.:

[Error]; Fatal Error: Steps limit reached: 10

There is no reason for considering the reaching of the steps limit as a "fatal error" or even an error. It should instead report a regular unknown or I don't know result, as it does when the time limit is reached

Moreover, it would be better to comply to the more or less SMT compliant answer, where one gets extra information using (get-info :reason-unknown)

[bclement-ocp]

We have added support for (get-info :reason-unknown) in #853 which will be part of the next release.

However it looks like we still treat the steps limit as an error (although it is now in stm2 format, which is an improvement I guess), so that part still needs to be fixed (we should treat steps limit like a timeout, it shouldn't be part of the Errors module):

(error "Fatal Error: Steps limit reached: 1")