replace os.Exit with panic for logger.CriticalIf (#6065)

This commit prevents complete server failures caused by
`logger.CriticalIf` calls. Instead of calling `os.Exit(1)`
the function now executes a panic with a special value
indicating that a critical error happend. At the top HTTP
handler layer panics are recovered and if its a critical
error the client gets an InternalServerError status code.

Further this allows unit tests to cover critical-error code
paths.

cmd/logger/logger.go

@@ -362,13 +362,15 @@ func LogIf(ctx context.Context, err error) {
 	fmt.Println(output)
 }
 
-// CriticalIf :
-// Like LogIf with exit
-// It'll be called for fatal error conditions during run-time
+// ErrCritical is the value panic'd whenever CriticalIf is called.
+var ErrCritical struct{}
+
+// CriticalIf logs the provided error on the console. It fails the
+// current go-routine by causing a `panic(ErrCritical)`.
 func CriticalIf(ctx context.Context, err error) {
 	if err != nil {
 		LogIf(ctx, err)
-		os.Exit(1)
+		panic(ErrCritical)
 	}
 }