Add sanity checks for contracts and coverageEnv folders

lib/app.js

@@ -57,6 +57,8 @@ class App {
     this.log('Generating coverage environment');
 
     try {
+      this.sanityCheckContext();
+
       let files = shell.ls(this.workingDir);
       const nmIndex = files.indexOf('node_modules');
 
@@ -188,7 +190,7 @@ class App {
       `
         There was an error generating coverage. Possible reasons include:
         1. Another application is using port ${this.port}
-        2. Truffle crashed because your tests errored
+        2. Your test runner (Truffle?) crashed because the tests encountered an error.
 
       `;
       this.cleanUp(msg + err);
@@ -240,6 +242,16 @@ class App {
 
   // ------------------------------------------ Utils ----------------------------------------------
 
+  sanityCheckContext(){
+    if (!shell.test('-e', `${this.workingDir}/contracts`)){
+      this.cleanUp("Couldn't find a 'contracts' folder to instrument.");
+    }
+
+    if (shell.test('-e', `${this.workingDir}/${this.coverageDir}`)){
+      shell.rm('-Rf', this.coverageDir);
+    }
+  }
+
   /**
    * Relativizes path keys so that istanbul report can be read on Windows
    * @param  {Object} map  coverage map generated by coverageMap