Add explanation to UnrecoverableKeyException

Unrecoverable keys after opening the keystore could be a consequence of
the key having a different password than the keystore itself.
This could cause some people to spend a lot of time figuring out what's
going on, so let's just give them a quick heads up of a potential
(common?) reason for this error.

This could be fixed by running "keytool -keypasswd".

src/core/src/main/java/org/apache/jmeter/util/SSLManager.java

@@ -157,7 +157,10 @@ protected synchronized JmeterKeyStore getKeyStore() {
            } catch (IOException e) {
                log.error("Can't load keystore '{}'. Wrong password?", fileName, e);
            } catch (UnrecoverableKeyException e) {
-               log.error("Can't recover keys from keystore '{}'", fileName, e);
+               log.error(
+                   "Can't recover keys from keystore '{}'. Is key password different from keystore password?",
+                   fileName,
+                   e);
            } catch (NoSuchAlgorithmException e) {
                log.error("Problem finding the correct algorithm while loading keys from keystore '{}'", fileName, e);
            } catch (CertificateException e) {