Added handling of exceptions when starting the HTTP server

Details:

* Added handling of IOError raised by start_http_server() in the HTTP case
  (it was already handled in the HTTPS case).

* For the HTTPS case, broadened the catching of IOError to now catch
  Exception, in order to make double sure that we catch everything, in order
  to investigate the issue with occasional ssl.SSLEOFError exceptions.

* Simplified and corrected the error message used for re-raising an exception
  raised by start_http_server() in the HTTPS case.

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

docs/changes.rst

@@ -65,6 +65,9 @@ Released: not yet
 
 * Test: Circumvented a pip-check-reqs issue by excluding its version 2.5.0.
 
+* Added handling of exceptions raised by the built-in HTTP server during
+  its startup, for the HTTP case. (related to issue #397)
+
 **Enhancements:**
 
 * Added support for Python 3.12. Had to increase the minimum versions of

zhmc_prometheus_exporter/zhmc_prometheus_exporter.py

@@ -1868,12 +1868,20 @@ def main():
                     keyfile=server_key_file,
                     client_cafile=ca_cert_file,
                     client_auth_required=(ca_cert_file is not None))
-            except IOError as exc:
+            # pylint_ disable=broad-exception
+            except Exception as exc:
+                # We catch Exception for now in order to investigate the
+                # issue that with ssl.SSLEOFError being raised occasionally.
                 raise ImproperExit(
-                    "Issues with server certificate, key, or CA certificate "
-                    "files: {}: {}".format(exc.__class__.__name__, exc))
+                    "Cannot start HTTPS server: {}: {}".
+                    format(exc.__class__.__name__, exc))
         else:
-            start_http_server(port=port)
+            try:
+                start_http_server(port=port)
+            except IOError as exc:
+                raise ImproperExit(
+                    "Cannot start HTTP server: {}: {}".
+                    format(exc.__class__.__name__, exc))
 
         logprint(logging.INFO, PRINT_ALWAYS,
                  "Exporter is up and running on port {}".format(port))