Catch and process possible exception on JSTransformer compileAsync and renderAsync

[ycherniavskyi]

Similarly to problem resolved by #20, because of uncached exception of JSTransformer compileAsync and renderAsync functions, the real problem does not output to console:

[13:57:21] Starting 'content'...
[13:57:22] The following tasks did not complete: content
[13:57:22] Did you forget to signal async completion?

Process finished with exit code 0

After applying this pull request in output we get more useful information (and correct exit code):

[15:31:09] Starting 'content'...
[15:31:10] 'content' errored after 1.06 s
[15:31:10] Error: ENOENT: no such file or directory, open '/Users/ych/projects/test/src/projects/extension/_idea.md'
    at /Users/ych/projects/test/src/projects/extension/index.html.pug line 10
    at Object.fs.openSync (fs.js:667:18)
    at Object.fs.readFileSync (fs.js:572:33)
    at Function.read (/Users/ych/projects/test/node_modules/pug-load/index.js:69:13)
    at Object.read (/Users/ych/projects/test/node_modules/pug/lib/index.js:147:25)
    at /Users/ych/projects/test/node_modules/pug-load/index.js:24:25
    at walkAST (/Users/ych/projects/test/node_modules/pug-walk/index.js:23:18)
    at /Users/ych/projects/test/node_modules/pug-walk/index.js:104:20
    at Array.reduce (<anonymous>)
    at walkAndMergeNodes (/Users/ych/projects/test/node_modules/pug-walk/index.js:103:18)
    at walkAST (/Users/ych/projects/test/node_modules/pug-walk/index.js:37:19)

Process finished with exit code 1

This exception throw because of include:markdown-it _idea.md Including Plain Text construction in index.html.pug.

After some investigation of the reason that async function throw exception I found next line in jstransformer implementation. So yes, it seems jstransformer issue, but looking at the date of its last release and count of unresolved issues, I think it will be easy and faster to fix it here.

In future, if the issue of compileAsync and renderAsync methods will be fixed this try/catch blocks could be deleted.

[RobLoach]

This is great! Thanks a lot. 👍