h5debug is unable to inspect file with broken root group

**Describe the bug**

When invalid root group or invalid superblock is detected, **h5debug** gives up early with "cannot open file”, and nothing else. This prevents detailed inspection of either superblock or root group, in scenarios when this exact capability is most needed. This behavior is confirmed by examining source code for **h5debug** and the **H5Fopen** function.

Please see [Forum: HDF5 binary difference](https://forum.hdfgroup.org/t/hdf5-binary-difference/13709) for a recent scenario. Due to confidentiality, you must acquire this interesting test file from the original reporter.

**Expected behavior**

Given a file name and internal object address, **h5debug** should make best effort to decode and display the file object at that address, regardless of invalid conditions anywhere else in the file.

**Platform (please complete the following information)**

- HDF5 version: As mentioned in the original conversation, both HDF5 1.4.5 and 2.1.0 have this behavior.
- OS and version: Any
- Compiler and version: Any
- Build system (e.g. CMake version) and generator (e.g. XCode, Ninja): Any