add message to '.Error()' output for LsError

[Diaphteiros]

How to categorize this PR?

/area logging
/kind enhancement
/priority 3

What this PR does / why we need it:
When wrapping an error in an object of type LsError, a message can be specified. Calling .Error() on the LsError object returns only the original error message, though. If this LsError object is then again wrapped, neither .Message, nor .Error() called on the new object will return the message specified during the first wrapping. This results in a loss of potentially interesting information.

To fix this, the .Error() implementation of LsError has been enhanced in the LsError type: Instead of just returning .Error() of the internal error, it will now return a combination of .Message and .Error(). It also checks if former one already contains the latter one, to avoid duplicate errors.

Release note:

Calling `.Error()` on a `LsError` object will now return a combination of the LsError's message and its internal error, instead of just returning the internal error's message.

[Diaphteiros]

cc @reshnm

[gardener-robot]

@In-Ko You have pull request review open invite, please check

[Diaphteiros]

/test

[testmachinery]

Testrun: e2e-njlmt
Workflow: e2e-njlmt-wf
Phase: Failed

+--------------------+--------------------+-----------+----------+
|        NAME        |        STEP        |   PHASE   | DURATION |
+--------------------+--------------------+-----------+----------+
| create-cluster     | create-cluster     | Succeeded | 3m11s    |
| create-registry    | create-registry    | Succeeded | 2m41s    |
| install-landscaper | install-landscaper | Succeeded | 44s      |
| integration-test   | test               | Failed    | 6m4s     |
| delete-cluster     | delete-cluster     | Omitted   | 0s       |
| delete-registry    | delete-registry    | Omitted   | 0s       |
+--------------------+--------------------+-----------+----------+

[achimweigel]

/lgtm