Warn in Gradle log when incremental compilation fails

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

compiler/daemon/daemon-common/src/org/jetbrains/kotlin/daemon/common/CompilerServicesFacadeBase.kt

@@ -48,8 +48,9 @@ enum class ReportSeverity(val code: Int) {
     DEBUG(3);
 
     companion object {
-        fun fromCode(code: Int): ReportSeverity? =
-                ReportSeverity.values().firstOrNull { it.code == code }
+        fun fromCode(code: Int): ReportSeverity {
+            return values().firstOrNull { it.code == code } ?: error("Can't find a matching ReportSeverity with code = $code")
+        }
     }
 }