fix: exit with code 1 when report output fails

[shinnn]

Currently, when c8 fails to create coverage reports because of, for example, unknown report type --reporter=unknown, it causes the following two problems:

1. The error is displayed as an UnhandledPromiseRejectionWarning to end users.

$ c8 --reporter=unknown echo ""

(node:26891) UnhandledPromiseRejectionWarning: Error: Cannot find module 'unknown'
Require stack:
- /Users/shinnn/c8/node_modules/istanbul-reports/index.js
- /Users/shinnn/c8/lib/report.js
- /Users/shinnn/c8/lib/commands/check-coverage.js
- /Users/shinnn/c8/lib/commands/report.js
- /Users/shinnn/c8/bin/c8.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:610:15)
    at Function.Module._load (internal/modules/cjs/loader.js:526:27)
    at Module.require (internal/modules/cjs/loader.js:666:19)
    at require (internal/modules/cjs/helpers.js:16:16)
    at Object.create (/Users/shinnn/c8/node_modules/istanbul-reports/index.js:14:20)
    at /Users/shinnn/c8/lib/report.js:47:26
    at Array.forEach (<anonymous>)
    at Report.run (/Users/shinnn/c8/lib/report.js:46:19)
    at processTicksAndRejections (internal/process/task_queues.js:89:5)
    at async exports.outputReport (/Users/shinnn/c8/lib/commands/report.js:24:3)
(node:26891) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:26891) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

2. c8 exits with code 0.

$ echo $?
0

This patch solves those problems by explicitly catching report creation errors and exiting with 1.

Checklist

• npm test, tests passing
• tests and/or benchmarks are included

[coveralls]

Pull Request Test Coverage Report for Build 252

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 96.667%

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Totals
Change from base Build 248:	0.06%
Covered Lines:	438
Relevant Lines:	447

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💛 - Coveralls

[bcoe]

👍 good catch.