[#89] cppcms_error fix typo + consistent messages

[dreaming-augustin]

This fixes the typo in #89 and adds some consistency in the error messages (punctuation + spacing).

[artyom-beilis]

I think I'll move master to 1x branch and I'll merge cpp11 branch to master.

cpp11 is 2.0 beta... it is time to make it main

[artyom-beilis]

Done... now 2.0 is master - I merged. It includes your fix as well of course

[dreaming-augustin]

Great. Thank you.