Fix crash on very long error messages

Fix crash on very long error messages
Manual for snprintf says:
       If  the output was truncated, the return value is -1, oth-
       erwise it is the number of characters stored, not  includ-
       ing the terminating null.
And that's a blatant lie - in reality, libc 2.1 always returns number of
characters that _would be_ stored. I hate those libc bugs. Now we should go
and check every place we trusted snprintf return value.