Show nice error when attempting to ng-add something that doesn't exist

[StephenFluin]

Versions

Angular CLI: 6.0.0-rc.6

Repro steps

ng new my-app
cd my-app
ng add @angular/material@5.2.9

Observed behavior

Installed packages for tooling via npm.
Cannot read property 'startsWith' of undefined
TypeError: Cannot read property 'startsWith' of undefined
    at NodeModulesEngineHost._resolvePath (/super/workspace/test/material-test/node_modules/@angular-devkit/schematics/tools/node-module-engine-host.js:31:18)
    at NodeModulesEngineHost._resolveCollectionPath (/super/workspace/test/material-test/node_modules/@angular-devkit/schematics/tools/node-module-engine-host.js:69:35)
    at NodeModulesEngineHost.createCollectionDescription (/super/workspace/test/material-test/node_modules/@angular-devkit/schematics/tools/file-system-engine-host-base.js:106:27)
    at SchematicEngine._createCollectionDescription (/super/workspace/test/material-test/node_modules/@angular-devkit/schematics/src/engine/engine.js:78:40)
    at SchematicEngine.createCollection (/super/workspace/test/material-test/node_modules/@angular-devkit/schematics/src/engine/engine.js:71:43)
    at Object.getCollection (/super/workspace/test/material-test/node_modules/@angular/cli/utilities/schematics.js:28:31)
    at AddCommand.getOptions (/super/workspace/test/material-test/node_modules/@angular/cli/models/schematic-command.js:187:41)
    at AddCommand.<anonymous> (/super/workspace/test/material-test/node_modules/@angular/cli/commands/add.js:29:49)
    at Generator.next (<anonymous>)
    at /super/workspace/test/material-test/node_modules/@angular/cli/commands/add.js:7:71

Desired behavior

Show a friendly error message that we couldn't find any schematics in the referenced package.