Fixes for the server reachability test.

- Do not apply HTTPs redirection for challenge used by the test.
- Set the `User-Agent` to avoid 403 answer from site24x7.com.
- Handle JSON parsing failure of the received body.
- Better handling of different error cases.

backend/internal/certificate.js

@@ -1167,6 +1167,7 @@ const internalCertificate = {
 			const options  = {
 				method:  'POST',
 				headers: {
+					'User-Agent':     'Mozilla/5.0',
 					'Content-Type':   'application/x-www-form-urlencoded',
 					'Content-Length': Buffer.byteLength(formBody)
 				}
@@ -1179,12 +1180,22 @@ const internalCertificate = {
 
 					res.on('data', (chunk) => responseBody = responseBody + chunk);
 					res.on('end', function () {
-						const parsedBody = JSON.parse(responseBody + '');
-						if (res.statusCode !== 200) {
-							logger.warn(`Failed to test HTTP challenge for domain ${domain}`, res);
+						try {
+							const parsedBody = JSON.parse(responseBody + '');
+							if (res.statusCode !== 200) {
+								logger.warn(`Failed to test HTTP challenge for domain ${domain} because HTTP status code ${res.statusCode} was returned: ${parsedBody.message}`);
+								resolve(undefined);
+							} else {
+								resolve(parsedBody);
+							}
+						} catch (err) {
+							if (res.statusCode !== 200) {
+								logger.warn(`Failed to test HTTP challenge for domain ${domain} because HTTP status code ${res.statusCode} was returned`);
+							} else {
+								logger.warn(`Failed to test HTTP challenge for domain ${domain} because response failed to be parsed: ${err.message}`);
+							}
 							resolve(undefined);
 						}
-						resolve(parsedBody);
 					});
 				});
 
@@ -1198,6 +1209,9 @@ const internalCertificate = {
 			if (!result) {
 				// Some error occurred while trying to get the data
 				return 'failed';
+			} else if (result.error) {
+				logger.info(`HTTP challenge test failed for domain ${domain} because error was returned: ${result.error.msg}`);
+				return `other:${result.error.msg}`;
 			} else if (`${result.responsecode}` === '200' && result.htmlresponse === 'Success') {
 				// Server exists and has responded with the correct data
 				return 'ok';