Provides an empty png instead of an error when imagemagick is not found

[paulmelnikow]

Generating e.g. .png badges from the CLI or API will fail silently when imagemagick is not installed. An empty .png is emitted.

Expected behavior from the CLI: an error return status and an error message. From the API: an error message in the response, a 503 status code, and a log message.

This was a gotcha for me, and would be for other people getting set up as well: see #883.

The issue is upstream issue in gm. When spawning convert, this line invokes the callback immediately, before the subprocess has a chance to emit the error.

Reported at aheckmann/gm#548 aheckmann/gm#256 but there hasn't been a fix. Not much active maintenance on that project, unfortunately.

Some possible resolutions:

1. Fix upstream (seems best)
2. Raise an error when nothing gets emitted (seems a serviceable hack)
3. Run a quick self-test when the server starts up (seems worst)

[paulmelnikow]

Opened a patch upstream aheckmann/gm#647, though it may not be merged timely.