seastar-addr2line: don't print an empty line between backtrace lines

In most cases addr2line's output will already has a newline character at
the end of the last line (because the line coming from the input does),
no need for `print()` to add another one, introducing an unnecessary
empty line. Use `sys.stdout.write()` to have full control over when a
new line is printed.

Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <5481db11ef4cf50f07805f26005349c22c56a797.1535026792.git.bdenes@scylladb.com>

execution_stage: fix wrong exception thrown for non-unique stages

The metrics API now also enforces uniqueness, and as it is called before
execution stage registration, it throws earlier, and confuses the unit test
because a different exception type is expected.

Fix by reversing the order of metrics registration and execution stage
registration.