Misleading exception is throw when executing test with a non-existing command

When executing tests with a non-existing command (e.g. `lein cljsbuild test does-not-exist`) following exception is thrown:

```java
Running ClojureScript test: does-not-exist
java.lang.IllegalArgumentException: No matching method found: exec for class java.lang.Runtime
	at clojure.lang.Reflector.invokeMatchingMethod(Reflector.java:80)
	at clojure.lang.Reflector.invokeInstanceMethod(Reflector.java:28)
	at cljsbuild.util$process_start.invoke(util.clj:76)
	at cljsbuild.util$sh.invoke(util.clj:86)
	at cljsbuild.test$run_tests$iter__20__24$fn__25.invoke(test.clj:16)
	at clojure.lang.LazySeq.sval(LazySeq.java:42)
	at clojure.lang.LazySeq.seq(LazySeq.java:60)
	at clojure.lang.RT.seq(RT.java:484)
	at clojure.core$seq.invoke(core.clj:133)
	at clojure.core$dorun.invoke(core.clj:2780)
	at clojure.core$doall.invoke(core.clj:2796)
	at cljsbuild.test$run_tests.invoke(test.clj:15)
	at user$eval5.invoke(NO_SOURCE_FILE:1)
	at clojure.lang.Compiler.eval(Compiler.java:6619)
	at clojure.lang.Compiler.eval(Compiler.java:6609)
	at clojure.lang.Compiler.eval(Compiler.java:6582)
	at clojure.core$eval.invoke(core.clj:2852)
	at clojure.main$eval_opt.invoke(main.clj:308)
	at clojure.main$initialize.invoke(main.clj:327)
	at clojure.main$null_opt.invoke(main.clj:362)
	at clojure.main$main.doInvoke(main.clj:440)
	at clojure.lang.RestFn.invoke(RestFn.java:421)
	at clojure.lang.Var.invoke(Var.java:419)
	at clojure.lang.AFn.applyToHelper(AFn.java:163)
	at clojure.lang.Var.applyTo(Var.java:532)
	at clojure.main.main(main.java:37)
```