Don't write to stderr when there are no errors

[tylerbutler]

tsc-multi writes to stderr even when it just says "Found 0 errors". It should use stdout in the success case.

[niieani]

The unix philosophy generalizes stderr for writing information about errors, not necessarily just errors themselves. Check out how tools like curl use stderr for diagnostic output (progress meters, and other diagnostic information all go to stderr).
stdout is usually reserved for output that is meant to be piped to other tools.

From wikipedia:

Standard error is another output stream typically used by programs to output error messages or diagnostics.

I think the current behavior is fine, since "Found 0 errors" qualifies as a diagnostic message in my view.