[OB3] Return meaningful error messages from APIM Devportal

[KalanaDananjaya]

Description:
When an error happens in APIM devportal, it shows a generic error like "Key Management error" in APIM dev portal. This is very generic and the user cannot identify the cause for the issue by looking at this error.
To fix this, APIM devportal's frontend code should be changed to show the error description sent from the backend instead of the error message field.

Further, the backend returns a set of predefined set of errors. Since we are now doing the input validations in the backend, it is better if we can show the exact error on the frontend(show which validation failed instead of showing "Key Management error". To do this APIM backend code should be changed to return the exact error instead of sending the predefined errors. ATM the error message we are setting from the backend and a predefined error is substituted before sending the response to the backend. (https://github.com/wso2/carbon-apimgt/blob/master/components/apimgt/org.wso2.carbon.apimgt.api/src/main/java/org/wso2/carbon/apimgt/api/APIManagementException.java#L42).

Affected Product Version:
OB3