update beta error messages and checks

[ryanelandt]

This is chunk 2 for #1000.

Runtime errors
One approach to run-time errors takes the form:

if(!(boost::math::isfinite)(a))
   return policies::raise_domain_error<T>(function, "a is bad because reason (got a=%1%).", a, pol);

When using this approach, it's understood that the error message should agree with the check that is actually being performed.

This PR
This PR fixes 6 places where the check and stated error message lacked agreement in the beta.hpp file. These incorrect error messages make debugging the Newton solver take longer than it would have otherwise.

Ideas about future solutions
Disagreement between the check and the error message are too easy to make. I think a better solution on a high level would to be able to use the BOOST_TEST types macros for run-time checks. I don't know if this kind of thing is possible. But either way, that's out of the scope of this PR.

[mborland]

LGTM. You are correct that there are probably other places where other incorrect error messages are lingering. Thanks.

[ryanelandt]

Thanks!