Added logError function (Closes #121)

[tomsotte]

Closes #121

Add an handy logError function that pretty-prints the error.
It emits 'end' to prevent exiting gulp abruptly. This is particularly useful for watching task, to not end them on every error.

---

Output example when compiling a .js file with an error:

With logError:

[13:20:46] Using gulpfile D:\workspace\source\test-gulp-babel\gulpfile.js
[13:20:46] Starting 'babel'...
SyntaxError in plugin 'gulp-babel'
Message:
    D:\workspace\source\test-gulp-babel\index.error.js: Unexpected token (2:0)
  1 | var a =
> 2 |
    | ^
[13:20:46] Finished 'babel' after 426 ms
[13:20:46] Starting 'after-babel'...
[13:20:46] Finished 'after-babel' after 5.37 ms

Without logError:
Notice that this doesn't continue on the after-babel task.

[13:22:14] Using gulpfile D:\workspace\source\test-gulp-babel\gulpfile.js
[13:22:14] Starting 'babel'...

events.js:160
      throw er; // Unhandled 'error' event
      ^
SyntaxError: D:\workspace\source\test-gulp-babel\index.error.js: Unexpected token (2:0)
  1 | var a =
> 2 |
    | ^
    at Parser.raise (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:807:15)
    at Parser.unexpected (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:2068:16)
    at Parser.parseExprAtom (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:3120:20)
    at Parser.parseExprSubscripts (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:2770:21)
    at Parser.parseMaybeUnary (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:2748:21)
    at Parser.parseExprOps (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:2671:21)
    at Parser.parseMaybeConditional (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:2647:21)
    at Parser.parseMaybeAssign (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:2607:21)
    at Parser.parseVar (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:4508:26)
    at Parser.parseVarStatement (D:\workspace\source\gulp-babel\node_modules\babylon\lib\index.js:4324:10)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! test-gulp-babel@1.0.0 start: `gulp after-babel`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the test-gulp-babel@1.0.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     C:\Users\tommaso\AppData\Roaming\npm-cache\_logs\2017-10-13T11_22_14_707Z-debug.log

[Lemmmy]

Any updates on this PR?

[loganfsmyth]

I'm not sure having this be part of gulp-babel makes sense. It's a general utility function for gulp streams, so it seems like it would be better in a utility package, if it's not already available.

[tomsotte]

I see your point, it may be best as an external utility package.
At the time I haven't found one and gulp-sass had this function that was helpful in logging the errors without interrupting the watch.

[loganfsmyth]

I was probably being overly aggressive, I don't really have an issue with landing this if you want. It will need to be tweaked and rebased for Babel 7 though. err.codeFrame doesn't exist anymore, so it'll literally be just stringifying the err.