Fix #536: For Resolve error report err.stack

Resolve errors are most likely caused by invalid configuration,
and the reason is easier to determine with the full details
rather than just `err.message`.

See #536

With this change, it reports something like:
```
import/no-unresolved: Resolve error: SyntaxError: Unexpected token import
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:387:25)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Module.require (module.js:367:17)
    at require (internal/module.js:16:19)
    at module.exports (/__censored__/webpack/configFactory.js:216:3)
    at configProdClient (/__censored__/webpack/configProdClient.js:5:36)
    at Object.<anonymous> (/__censored__/webpack/configForEslintImportResolver.js:1:126)
```

tests/files/load-error-resolver.js

@@ -1 +1 @@
-throw new Error('TEST ERROR')
+throw new SyntaxError('TEST SYNTAX ERROR')

tests/src/core/resolve.js

@@ -8,6 +8,11 @@ import * as fs from 'fs'
 import * as utils from '../utils'
 
 describe('resolve', function () {
+  // We don't want to test for a specific stack, just that it was there in the error message.
+  function replaceErrorStackForTest(str) {
+    return typeof str === 'string' ? str.replace(/(\n\s+at .+:\d+\)?)+$/, '\n<stack-was-here>') : str
+  }
+
   it('throws on bad parameters', function () {
     expect(resolve.bind(null, null, null)).to.throw(Error)
   })
@@ -60,7 +65,7 @@ describe('resolve', function () {
                       , Object.assign({}, testContext, { getFilename: function () { return utils.getFilename('exception.js') } })
                     )).to.equal(undefined)
     expect(testContextReports[0]).to.be.an('object')
-    expect(testContextReports[0].message).to.equal('Resolve error: foo-bar-resolver-v2 resolve test exception')
+    expect(replaceErrorStackForTest(testContextReports[0].message)).to.equal('Resolve error: foo-bar-resolver-v2 resolve test exception\n<stack-was-here>')
     expect(testContextReports[0].loc).to.eql({ line: 1, column: 0 })
 
     testContextReports.length = 0
@@ -145,7 +150,7 @@ describe('resolve', function () {
                       , Object.assign({}, testContext, { getFilename: function () { return utils.getFilename('exception.js') } })
                     )).to.equal(undefined)
     expect(testContextReports[0]).to.be.an('object')
-    expect(testContextReports[0].message).to.equal('Resolve error: TEST ERROR')
+    expect(replaceErrorStackForTest(testContextReports[0].message)).to.equal('Resolve error: SyntaxError: TEST SYNTAX ERROR\n<stack-was-here>')
     expect(testContextReports[0].loc).to.eql({ line: 1, column: 0 })
   })
 

utils/resolve.js

@@ -146,7 +146,9 @@ function resolverReducer(resolvers, map) {
     return map
   }
 
-  throw new Error('invalid resolver config')
+  const err = new Error('invalid resolver config')
+  err.name = 'EslintPluginImportResolveError'
+  throw err
 }
 
 function getBaseDir(sourceFile) {
@@ -159,10 +161,14 @@ function requireResolver(name, sourceFile) {
     tryRequire(path.resolve(getBaseDir(sourceFile), name))
 
   if (!resolver) {
-    throw new Error(`unable to load resolver "${name}".`)
+    const err = new Error(`unable to load resolver "${name}".`)
+    err.name = 'EslintPluginImportResolveError'
+    throw err
   }
   if (!isResolverValid(resolver)) {
-    throw new Error(`${name} with invalid interface loaded as resolver`)
+    const err = new Error(`${name} with invalid interface loaded as resolver`)
+    err.name = 'EslintPluginImportResolveError'
+    throw err
   }
 
   return resolver
@@ -194,8 +200,14 @@ function resolve(p, context) {
                    )
   } catch (err) {
     if (!erroredContexts.has(context)) {
+      // The `err.stack` string starts with `err.name` followed by colon and `err.message`.
+      // We're filtering out the default `err.name` because it adds little value to the message.
+      let errMessage = err.message
+      if (err.name !== 'EslintPluginImportResolveError' && err.stack) {
+        errMessage = err.stack.replace(/^Error: /, '')
+      }
       context.report({
-        message: `Resolve error: ${err.message}`,
+        message: `Resolve error: ${errMessage}`,
         loc: { line: 1, column: 0 },
       })
       erroredContexts.add(context)