Improve `SentryException#value`, remove stringified stack trace

[marandaneto]

Description

sentry-dart/dart/lib/src/sentry_exception_factory.dart

Line 64 in 211a7aa

value: throwable.toString(),

Some exceptions return the stringified stack trace when calling toString, this makes an ugly/not-so-useful/minified issue's message.
Doing heuristics on line breaks and regexing/matching a stack trace.

[marandaneto]

Example if split debug symbols are enabled:

NetworkError(type: NetworkErrorType.unknown, error: Instance of 'iH')

---

pid: 9437, tid: 10069, name 1.ui
os: android arch: arm64 comp: yes sim: no
build_id: 'bca64abfdfcc84d231bb8f1ccdbfbd8d'
isolate_dso_base: 752602b000, vm_dso_base: 752602b000
isolate_instructions: 7526344980, vm_instructions: 752633f000
#00 abs 00000075266c2fbf virt 0000000000697fbf _kDartIsolateSnapshotInstructions+0x37e63f
#1 abs 000000752685211f virt 000000000082711f _kDartIsolateSnapshotInstructions+0x50d79f
#2 abs 0000007526851cb3 virt 0000000000826cb3 _kDartIsolateSnapshotInstructions+0x50d333
#3 abs 0000007526851c63 virt 0000000000826c63 _kDartIsolateSnapshotInstructions+0x50d2e3
#4 abs 0000007526851bf3 virt 0000000000826bf3 _kDartIsolateSnapshotInstructions+0x50d273
#5 abs 0000007526a0b44b virt 00000000009e044b _kDartIsolateSnapshotInstructions+0x6c6acb
#6 abs 0000007526a068a7 virt 00000000009db8a7 _kDartIsolateSnapshotInstructions+0x6c1f27
#7 abs 0000007526b57a2b virt 0000000000b2ca2b _kDartIsolateSnapshotInstructions+0x8130ab
#8 abs 0000007526b5d93b virt 0000000000b3293b _kDartIsolateSnapshotInstructions+0x818fbb
#9 abs 0000007526a2333b virt 00000000009f833b _kDartIsolateSnapshotInstructions+0x6de9bb
#10 abs 0000007526937957 virt 000000000090c957 _kDartIsolateSnapshotInstructions+0x5f2fd7
#11 abs 0000007526a243a3 virt 00000000009f93a3 _kDartIsolateSnapshotInstructions+0x6dfa23
#12 abs 000000752636273b virt 000000000033773b _kDartIsolateSnapshotInstructions+0x1ddbb
#13 abs 0000007526a36ac3 virt 0000000000a0bac3 _kDartIsolateSnapshotInstructions+0x6f2143
#14 abs 00000075263626af virt 00000000003376af _kDartIsolateSnapshotInstructions+0x1dd2f