teuthology/scrape: Fix bad backtrace parsing in Teuthology.log

teuthology/scrape: Fix bad backtrace parsing in Teuthology.log

Problem:

- confusing warning message stating that
the back trace is malformed

- We kept adding to the backtrace buffer
even when we exceeded the `MAX_BT_LINES`

Solution:

- Correct the warning message to be
"Ignoring backtrace that exceeds MAX_BT_LINES"
- reset the buffer once we exceeded MAX_BT_LINES
- Added some cases where we detect start/end of back trace.

Fixes:https://tracker.ceph.com/issues/62445

Signed-off-by: Kamoltat Sirivadhna <ksirivad@redhat.com>