Processing Errors are now Stored in the ElasticSearch Index

[johannesduesing]

Reason for this PR
As decribed in #31, the Delphi Crawler does not keep track of any errors that may occur during processing of artifacts. If for example the JAR download fails for any reason, the respective artifact will not be stored in the ElasticSearch index, and there is no way of retrying the download at a later point in time.

Changes in this PR (changes relative to #47, needs to be merged first!)
The MavenDiscoveryProcess now accounts for different types of processing errors. These errors are caught and redirected to a dedicated actor, which stores them as a new mapping type (MavenProcessingError) in the existing ElasticSearch index named delphi. The initial database setup has been adapted so that the new mapping type is created on startup (for fresh DB instances only!). A MavenProcessingError has four properties:

• type: Either PomDownloadFailed, JarDownloadFailed, PomParsingFailed or HermesProcessingFailed (It must be noted that, unlike in previous versions of the crawler, a failed JAR download is no longer an error if the POM file's packaging attribute is not set to jar)
• occurred: Datetime at which the error occurred
• identifier: Maven identifier referencing the artifact for which the error occurred
• message: An error message that describes the error cause

Closes #31.

Sample ElasticSearch Request
GET <elasticsearch>/delphi/error/_search

"hits":[
         {
            "_index":"delphi",
            "_type":"error",
            "_id":"1603115282027",
            "_score":1.0,
            "_source":{
               "identifier":{
                  "groupId":"yan",
                  "artifactId":"yan",
                  "version":"5.0"
               },
               "occurred":"2020-10-19T15:48:02.027+02:00",
               "message":"Got an unexpected HTTP response, code 404 Not Found.",
               "type":"PomDownloadFailed"
            }
         },
         {
            "_index":"delphi",
            "_type":"error",
            "_id":"1603115325347",
            "_score":1.0,
            "_source":{
               "identifier":{
                  "groupId":"yan",
                  "artifactId":"jfunutil",
                  "version":"5.0.2"
               },
               "occurred":"2020-10-19T15:48:45.347+02:00",
               "message":"Got an unexpected HTTP response, code 404 Not Found.",
               "type":"PomDownloadFailed"
            }
         },[...]
]

[sonarqubecloud]

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[johannesduesing]

Closed as this functionality is now part of the redesign proposed in #50