fix: nyc path subtly different with new yarn, require relative to cwd

this should be backwards-compatible with the previous lookup style, but
also works correctly with new yarn

if not, and there is a problem (either with existing users or in the future)
it now attempts to print out a helpful message indicating why it failed and
where it looked

jet.js

@@ -1,7 +1,16 @@
 #!/usr/bin/env node
+const fs = require('fs');
+
 if (process.argv.includes('--coverage') && !process.env.NYC_CONFIG) {
-  process.argv.unshift(require.resolve('./node_modules/.bin/nyc'));
-  require('./node_modules/.bin/nyc');
+  const nycPath = `${process.cwd()}/node_modules/.bin/nyc`;
+  if (!fs.existsSync(nycPath)) {
+    throw Error(
+      `Jet: '--coverage' requested but cannot find 'nyc'. Did not run from app root? Looked in: '${nycPath}'`
+    );
+  }
+
+  process.argv.unshift(require.resolve(nycPath));
+  require(nycPath);
 } else {
   require('./lib/commonjs/cli.js');
 }