Use relative paths for the end of compile report

[orta]

In 4.6, when a compile ends and there is more than one file with an error, we present something like:

Found 4 errors in 4 files.

Errors  Files
     1  /path/to/typescript/src/compiler/watch.ts:487
     1  /path/to/typescript/src/services/codefixes/requireInTs.ts:54
     1  /path/to/typescript/src/services/globalThisShim.ts:9
     1  /path/to/typescript/src/testRunner/externalCompileRunner.ts:4

This PR changes it to:

Found 4 errors in 4 files.

Errors  Files
     1  src/compiler/watch.ts:487
     1  src/services/codefixes/requireInTs.ts:54
     1  src/services/globalThisShim.ts:9
     1  src/testRunner/externalCompileRunner.ts:4

Based on your current working directory.