Fix a crash when gdb produces malformed output.

Summary:
Fix a crash when gdb produces malformed output. The crash was reported in https://bugs.kde.org/show_bug.cgi?id=376595.

It is actually a bug in gdb which can't handle invalid/out-of-scope watchpoints gracefully.

Instead of assert and crash the whole IDE, debugger errors are now reported to the user in a dialog.
And debug session will automatically end in this case.

Test Plan: A minimal test case from the reporter: https://bugs.kde.org/show_bug.cgi?id=376595#c20

Reviewers: apol, #kdevelop, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4714