Improve error message if an import cannot be found

[dcodeIO]

This PR fixes #1282 by adding the paths searched to the error message when an imported file cannot be found, for example when trying to import a file by erroneously specifying a file extension, looking like:

ERROR: Import 'tests/compiler/404.ts' not found.
  No such file 'path\to\tests\compiler\404.ts.ts'
  No such file 'path\to\tests\compiler\404.ts\index.ts'
  No such file 'path\to\tests\compiler\404.ts.d.ts'

Doesn't implement a .ts specific error message because that might be intended, but the problem should be clear from the paths searched. cc @torch2424

[torch2424]

Oh yay! Thanks for fixing this! 😄

I know we recently added a flag so you can specify a file extension other than .ts. Would this be affected by that? Would it be easy to account for that flag here? 🤔

[dcodeIO]

Oh, right, good catch. Will fix :)

[dcodeIO]

Now the error looks like

ERROR: Import path 'assembly/404.as' in 'assembly/index' not found.
  No such file 'path\to\assembly\404.as.as'
  No such file 'path\to\assembly\404.as\index.as'
  No such file 'path\to\assembly\404.as.d.as'

on

import "./404.as";

using --extension .as.

[dcodeIO]

Last commit goes a little further by moving the error reporting for files not being found to the compiler, by accepting a null text in parseFile. Has the advantage that the respective code region can be highlighted in a diagnostic, which is something I always wanted to do. A typical error now looks like ERROR TS6054: File '404.ts.ts' not found.. Doesn't display search paths, but by highlighting the code location this should actually be even clearer.

[torch2424]

Implementation LGTM 😄 But, I don't know the compiler super well, but from what I do understand, the code makes sense and looks good 😄