Preserve whitespace in traceback text

Exception messages sometimes contain detail information in separate lines, for readability in log/console output; this change preserves that readability in Sentry layout.

Example:

```
(TransactionRollbackError) deadlock detected DETAIL: Process 13410 waits for AccessExclusiveLock on relation 9923088 of database 9922762; blocked by process 13156. Process 13156 waits for AccessShareLock on relation 9923080 of database 9922762; blocked by process 13410. HINT: See server log for query details. '\nCREATE TABLE foo (\n\tfk_bar INTEGER NOT NULL, \n\tfk_baz INTEGER NOT NULL, \n\tfk_xyzzy INTEGER...
```

becomes:

```
(TransactionRollbackError) deadlock detected
DETAIL:  Process 13410 waits for AccessExclusiveLock on relation 9923088 of database 9922762; blocked by process 13156.
Process 13156 waits for AccessShareLock on relation 9923080 of database 9922762; blocked by process 13410.
HINT:  See server log for query details.
 '\nCREATE TABLE foo (\n\tfk_bar INTEGER NOT NULL, \n\tfk_baz INTEGER NOT NULL, \n\tfk_xyzzy INTEGER...
```
Merge pull request #1278 from gthb/patch-1

Preserve whitespace in traceback text