fix: catch possible SAML response signing error

[forrest-ua]

Description

This PR allows to catch an error that might happen due to invalid signing key (badly formatted) during SAML response signing.

Testing

• [x] This change adds test coverage for new/changed/fixed functionality

[forrest-ua]

I found that the same error happens in master branch, after some investigation it turns out the issue is caused by the new release of cheerio-select package. node-samlp is using cheerio@~0.10.7 to extract data from html page in the tests, the problem is that cheerio@~0.10.7 was using cheerio-select@* as dependency. And that made npm to load latest available cheerio-select@1.1.0 (MAJOR update released just recently) instead of previously used cheerio-select@0.0.3

I've pinned previosly used cheerio-select@0.0.3, as it's a dev dependency, and not to introduce more changes to this patch change.