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/README.md b/build/docker/README.md
index e3c2ec1..08023a7 100644
--- a/build/docker/README.md
+++ b/build/docker/README.md
@@ -178,7 +178,7 @@
 | dart      | 2.0.0         | 2.4.0         |       |
 | delphi    |               |               | Not in CI |
 | erlang    | 18.3          | 22.0          |       |
-| go        | 1.10.8        | 1.12.6        |       |
+| go        | 1.14.14       | 1.15.7        |       |
 | haskell   | 7.10.3        | 8.0.2         |       |
 | haxe      | 3.2.1         | 3.4.4         | THRIFT-4352: avoid 3.4.2 |
 | java      | 1.8.0_191     | 11.0.3        |       |
diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index 7deefcb..c8ecd8e 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -140,9 +140,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.13.1
+ENV GOLANG_VERSION 1.15.7
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 94f874037b82ea5353f4061e543681a0e79657f787437974214629af8407d124
+ENV GOLANG_DOWNLOAD_SHA256 0d142143794721bb63ce6c8a6180c4062bcf8ef4715e7d6d6609f3a8282629b3
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
             tar -C /usr/local -xzf golang.tar.gz && \
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 && \
diff --git a/build/docker/ubuntu-xenial/Dockerfile b/build/docker/ubuntu-xenial/Dockerfile
index 535db41..e554c53 100644
--- a/build/docker/ubuntu-xenial/Dockerfile
+++ b/build/docker/ubuntu-xenial/Dockerfile
@@ -137,9 +137,9 @@
       libglib2.0-dev
 
 # golang
-ENV GOLANG_VERSION 1.10.8
+ENV GOLANG_VERSION 1.14.14
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 d8626fb6f9a3ab397d88c483b576be41fa81eefcec2fd18562c87626dbb3c39e
+ENV GOLANG_DOWNLOAD_SHA256 6f1354c9040d65d1622b451f43c324c1e5197aa9242d00c5a117d0e2625f3e0d
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz && \
       echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - && \
       tar -C /usr/local -xzf golang.tar.gz && \