Fixed: UserMergeBranch loses error messages from the merge operation

The error handling block of mergeErr in UserMergeBranch inserted the err object into its fallback error message. The err is always nil here, this message should include mergeErr instead.

Also added more description to the internal errors in general, to make it possible to determine which of the errors was thrown.

An example of what the error message looked like before the fix:

{
...
"error":"%!w(\u003cnil\u003e)","grpc.code":"Internal",
"grpc.request.fullMethod":"/gitaly.OperationService/UserMergeBranch",
"level":"error",
"msg":"finished streaming call with code Internal"
...
}

After the fix:

{
...
"error":"merge: error parsing merge tree result"
"grpc.request.fullMethod":"/gitaly.OperationService/UserMergeBranch"
"level":"error",
"msg":"finished streaming call with code Internal"
...
}