fix: improve handling of disallowed formats (#429)

* fix: Fixes and tests for StopIteration on .convert()

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* fix: Remove unnecessary case handling

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* fix: Other test fixes

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* improve handling of unsupported types

- Introduced new explicit exception types instead of `RuntimeError`
- Introduced new `ConversionStatus` value for unsupported formats
- Tidied up converter member typing & removed asserts

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

* robustify & simplify format option resolution

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

* rename new status, populate ConversionResult errors

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

---------

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>
Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>
Co-authored-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>