[rust-server] Update to error-chain 0.12 (#395)

* Update to error-chain 0.12: 0.11 doesn't work with Rust 1.27.0 (new warnings)
* Revert commit 845df89

Author: bjgill

modules/openapi-generator/src/main/resources/rust-server/Cargo.mustache

@@ -47,4 +47,4 @@ uuid = {version = "0.5", optional = true, features = ["serde", "v4"]}
 
 [dev-dependencies]
 clap = "2.25"
-error-chain = "0.11"
+error-chain = "0.12"

pom.xml

@@ -959,7 +959,7 @@
                 <module>samples/client/petstore/typescript-angular-v4.3/npm</module>
                 <!--<module>samples/client/petstore/bash</module>-->
                 <module>samples/client/petstore/ruby</module>
-                <!--<module>samples/server/petstore/rust-server</module>-->
+                <module>samples/server/petstore/rust-server</module>
             </modules>
         </profile>
         <profile>

samples/server/petstore/rust-server/Cargo.toml

@@ -45,4 +45,4 @@ serde-xml-rs = {git = "git://github.com/Metaswitch/serde-xml-rs.git" , branch =
 
 [dev-dependencies]
 clap = "2.25"
-error-chain = "0.11"
+error-chain = "0.12"