THRIFT-5338: Raise minimal supported go version to 1.14.14

Client: go

- Update Dockerfiles used by travis
- Add go.mod file
- Modify error handling code to take advantage of errors package updates
  in go1.13
diff --git a/build/docker/ubuntu-disco/Dockerfile b/build/docker/ubuntu-disco/Dockerfile
index cfa4041..531718c 100644
--- a/build/docker/ubuntu-disco/Dockerfile
+++ b/build/docker/ubuntu-disco/Dockerfile
@@ -140,9 +140,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.12.6
+ENV GOLANG_VERSION 1.15.7
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 dbcf71a3c1ea53b8d54ef1b48c85a39a6c9a935d01fc8291ff2b92028e59913c
+ENV GOLANG_DOWNLOAD_SHA256 0d142143794721bb63ce6c8a6180c4062bcf8ef4715e7d6d6609f3a8282629b3
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
             tar -C /usr/local -xzf golang.tar.gz && \