When incremental compilation fails, we currently log it at the `debug`
level (and fall back to non-incremental compilation). This commit will
change it to `warning` so that we can get more user reports, which will
allow us to fix the root cause.
Also make sure the warning includes a stack trace.
Additionally, let ReportSeverity.fromCode() return a non-null value
or throw an exception otherwise as that case is not expected.
^KT-52839 In Progress
Hung Nguyen
e01c2bc651