infra: better error handling of invalid config file (#124)

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<!-- What changes are being made? (What feature/bug is being fixed here?) -->
**What**:
When a config file is loaded, but the contents cannot be parsed to JSON an error will be thrown. Instead of failing silent: e.g.:
<img width="735" alt="screen shot 2018-12-23 at 5 04 17 pm" src="https://user-images.githubusercontent.com/3534236/50382208-026cdb80-06d5-11e9-8f4d-8d97a9a3cbe1.png">


<!-- Why are these changes necessary? -->
**Why**:
Previously some commands would crash randomly. e.g. `add` with `Cannot read property 'then' of null`.
While other commands e.g. `generate` would succeed but would blow away all your contributors (because it didn't find any)

<!-- How were these changes implemented? -->
**How**:
By Me

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**Checklist**:
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- [ ] Documentation N/A
- [ ] Tests  N/A
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src/cli.js

@@ -40,7 +40,7 @@ const yargv = yargs
     try {
       return util.configFile.readConfig(configPath)
     } catch (error) {
-      if (configPath !== defaultRCFile) {
+      if (error instanceof SyntaxError || configPath !== defaultRCFile) {
         onError(error)
       }
     }

src/util/config-file.js

@@ -10,6 +10,9 @@ function readConfig(configPath) {
     }
     return config
   } catch (error) {
+    if (error instanceof SyntaxError) {
+      throw new SyntaxError(`Configuration file has malformed JSON: ${configPath}. Error:: ${error.message}`)
+    }
     if (error.code === 'ENOENT') {
       throw new Error(`Configuration file not found: ${configPath}`)
     }
@@ -24,7 +27,7 @@ function writeConfig(configPath, content) {
   if (!content.projectName) {
     throw new Error(`Error! Project name is not set in ${configPath}`)
   }
-  
+
   if (content.files && !content.files.length) {
     throw new Error(
       `Error! Project files was overridden and is empty in ${configPath}`,