Exit nicely when the compilation failed in Zinc

Currently, it fails with the following stacktrace : 
```
11:42:20.419 [ERROR] i.g.a.ZincCompiler$ - one error found
Exception in thread "main" Compilation failed
	at sbt.compiler.AnalyzingCompiler.call(AnalyzingCompiler.scala:105)
	at sbt.compiler.AnalyzingCompiler.compile(AnalyzingCompiler.scala:48)
	at sbt.compiler.AnalyzingCompiler.compile(AnalyzingCompiler.scala:41)
	at sbt.compiler.AggressiveCompile$$anonfun$3$$anonfun$compileScala$1$1.apply$mcV$sp(AggressiveCompile.scala:98)
	at sbt.compiler.AggressiveCompile$$anonfun$3$$anonfun$compileScala$1$1.apply(AggressiveCompile.scala:98)
	at sbt.compiler.AggressiveCompile$$anonfun$3$$anonfun$compileScala$1$1.apply(AggressiveCompile.scala:98)
	at sbt.compiler.AggressiveCompile.sbt$compiler$AggressiveCompile$$timed(AggressiveCompile.scala:155)
	at sbt.compiler.AggressiveCompile$$anonfun$3.compileScala$1(AggressiveCompile.scala:97)
	at sbt.compiler.AggressiveCompile$$anonfun$3.apply(AggressiveCompile.scala:138)
	at sbt.compiler.AggressiveCompile$$anonfun$3.apply(AggressiveCompile.scala:86)
	at sbt.inc.IncrementalCompile$$anonfun$doCompile$1.apply(Compile.scala:30)
	at sbt.inc.IncrementalCompile$$anonfun$doCompile$1.apply(Compile.scala:28)
	at sbt.inc.Incremental$.cycle(Incremental.scala:73)
	at sbt.inc.Incremental$$anonfun$1.apply(Incremental.scala:33)
	at sbt.inc.Incremental$$anonfun$1.apply(Incremental.scala:32)
	at sbt.inc.Incremental$.manageClassfiles(Incremental.scala:41)
	at sbt.inc.Incremental$.compile(Incremental.scala:32)
	at sbt.inc.IncrementalCompile$.apply(Compile.scala:25)
	at sbt.compiler.AggressiveCompile.compile2(AggressiveCompile.scala:146)
	at sbt.compiler.AggressiveCompile.compile1(AggressiveCompile.scala:70)
	at com.typesafe.zinc.Compiler.compile(Compiler.scala:161)
	at com.typesafe.zinc.Compiler.compile(Compiler.scala:142)
	at io.gatling.app.ZincCompiler$.main(ZincCompiler.scala:111)
	at io.gatling.app.ZincCompiler.main(ZincCompiler.scala)
Exception in thread "main" org.apache.commons.exec.ExecuteException: Process exited with an error: 1 (Exit value: 1)
	at org.apache.commons.exec.DefaultExecutor.executeInternal(DefaultExecutor.java:377)
	at org.apache.commons.exec.DefaultExecutor.execute(DefaultExecutor.java:160)
	at org.apache.commons.exec.DefaultExecutor.execute(DefaultExecutor.java:147)
	at io.gatling.app.ZincCompilerLauncher$.apply(ZincCompilerLauncher.scala:54)
	at io.gatling.app.SimulationClassLoader$.fromSourcesDirectory(SimulationClassLoader.scala:32)
	at io.gatling.app.Gatling$$anonfun$15.apply(Gatling.scala:171)
	at io.gatling.app.Gatling$$anonfun$15.apply(Gatling.scala:171)
	at scala.Option.getOrElse(Option.scala:120)
	at io.gatling.app.Gatling.start(Gatling.scala:171)
	at io.gatling.app.Gatling$.fromMap(Gatling.scala:59)
	at io.gatling.app.Gatling$.runGatling(Gatling.scala:80)
	at io.gatling.app.Gatling$.main(Gatling.scala:54)
	at io.gatling.app.Gatling.main(Gatling.scala)
```