print a meaninful error when string enums are used

[smola]

When a string type is used as a enum-like, proteus does generate code, but that code is invalid and fails to build.

For example:

➜  server git:(initial-version) ✗ go build ./cmd/bblfsh/...
# github.com/bblfsh/sdk/protocol
../sdk/protocol/generated.pb.go:233: cannot convert 0 to type Status
../sdk/protocol/generated.pb.go:233: invalid operation: m.Status != 0 (mismatched types Status and int)
../sdk/protocol/generated.pb.go:236: cannot convert m.Status (type Status) to type uint64
../sdk/protocol/generated.pb.go:305: cannot convert 0 to type Status
../sdk/protocol/generated.pb.go:305: invalid operation: m.Status != 0 (mismatched types Status and int)
../sdk/protocol/generated.pb.go:308: cannot convert m.Status (type Status) to type uint64
../sdk/protocol/generated.pb.go:378: cannot convert 0 to type Status
../sdk/protocol/generated.pb.go:378: invalid operation: m.Status != 0 (mismatched types Status and int)
../sdk/protocol/generated.pb.go:379: cannot convert m.Status (type Status) to type uint64
../sdk/protocol/generated.pb.go:407: cannot convert 0 to type Status
../sdk/protocol/generated.pb.go:407: too many errors

I saw it is actually documented in the README.md, but it would still be good to give an error or a warning when calling proteus.