Add error to output when module loaded as resolver has invalid API

tests/files/foo-bar-resolver-invalid.js

@@ -0,0 +1 @@
+exports = {};

tests/src/core/resolve.js

@@ -110,6 +110,22 @@ describe('resolve', function () {
     expect(testContextReports[0].loc).to.eql({ line: 1, column: 0 })
   })
 
+  it('reports loaded resolver with invalid interface', function () {
+    const resolverName = './foo-bar-resolver-invalid';
+    const testContext = utils.testContext({ 'import/resolver': resolverName });
+    const testContextReports = []
+    testContext.report = function (reportInfo) {
+      testContextReports.push(reportInfo)
+    }
+    testContextReports.length = 0
+    expect(resolve( '../files/foo'
+                      , Object.assign({}, testContext, { getFilename: function () { return utils.getFilename('foo.js') } })
+                    )).to.equal(undefined)
+    expect(testContextReports[0]).to.be.an('object')
+    expect(testContextReports[0].message).to.equal(`Resolve error: ${resolverName} with invalid interface loaded as resolver`)
+    expect(testContextReports[0].loc).to.eql({ line: 1, column: 0 })
+  })
+
   it('respects import/resolve extensions', function () {
     const testContext = utils.testContext({ 'import/resolve': { 'extensions': ['.jsx'] }})
 
@@ -119,7 +135,6 @@ describe('resolve', function () {
   })
 
   it('reports load exception in a user resolver', function () {
-
     const testContext = utils.testContext({ 'import/resolver': './load-error-resolver' })
     const testContextReports = []
     testContext.report = function (reportInfo) {

utils/resolve.js

@@ -160,8 +160,19 @@ function requireResolver(name, sourceFile) {
 
   if (!resolver) {
     throw new Error(`unable to load resolver "${name}".`)
+  }
+  if (!isResolverValid(resolver)) {
+    throw new Error(`${name} with invalid interface loaded as resolver`)
+  }
+
+  return resolver
+}
+
+function isResolverValid(resolver) {
+  if (resolver.interfaceVersion === 2) {
+    return resolver.resolve && typeof resolver.resolve === 'function'
   } else {
-    return resolver;
+    return resolver.resolveImport && typeof resolver.resolveImport === 'function'
   }
 }