Now sblint command exits with nonzero exit code

[svetlyak40wt]

This makes it more useful in CI pipelines.

Now it returns a number of found errors:

[art@poftheday] qlot exec sblint
src/appenders.lisp:41:0: style-warning: The variable B is defined but never used.
src/appenders.lisp:46:2: simple-warning: The function FOO is called with zero arguments, but wants exactly two.
src/appenders.lisp:50:2: type-warning: Constant 3.14 conflicts with its asserted type FIXNUM.
WARNING: Compilation failed in a system "log4cl-extras".
[art@poftheday] echo $?

[cxxxr]

Thank you.