Catch sourcemap JSON parse exception.

[masoncj]

@richardxia CC @jdavisp3

Catch and log exception on invalid sourcemap.

Tested by stopping in debugger, manually truncating sourcemap and continueing:

Warning: could not read valid sourcemap JSON from /Users/cmason/counsyl/code/civet/testsite/precompiled_assets/coffee/library.map. Exception is:
Expecting object: line 4 column 25 (char 66)

Contents are:
{
  "version": 3,
  "file": "library.js",
  "sourceRoot": "../..",

[richardxia]

Is it a good idea to open two file handles to the same file, one in read mode and one in write mode? I wasn't able to find anything on the internet about this, but it worries me slightly (is there a potential to self deadlock?).

Since you have a return statement in the the except case, you could just close the original file handle and open a new one, similar to the old code.

[masoncj]

Yeah, derp, thanks. 0a28db6

[richardxia]

Cool, LGTM!

[masoncj]

Thanks @richardxia!