Improve error messages when the dynatrace-service is unable to process an event

[arthurpitman]

Is your feature request related to a problem? Please describe the problem.
Currently the error messages generated by the dynatrace-service when it is unable to process an event are not very helpful.

Describe the solution you'd like
The error message should state that the dynatrace-service will not process the event and then give the reason:

• Event is missing a project
• Couldn't read dynatrace.conf.yaml configuration file
• Couldn't create a secret provider
• Couldn't read Dynatrace credentials
• Couldn't create a Keptn client