Invalid composer.json isn't handled gracefully

[gsteel]

Bug Report

This was first discovered in mezzio/mezzio-swoole#128

Summary

Matrix fails with unintelligible error when composer.json is invalid.

Expected behavior

Matrix generation should fail with an error message such as 'Cannot generate matrix. Invalid composer.json detected'

[boesing]

Before composer.json is parsed, we use JSON schema to validate.
Guess we just have to add some error handling around that to catch invalid JSON files in general.