Handle node-fetch text parse error

[upperwalker]

Hi,

Was investigating issue which caused our application to crash from time to time.
It turned out that CT SDK client can produce Unhandled promise rejection when parsing response text.

Steps to reproduce:
Executed 50-100 parallel requests to fetch order by id in endless loop, error pops up randomly within first 1-3 minutes:

• Error: Invalid response body while trying to fetch ...: read ECONNRESET

Also we've noticed that it can cause similar error, which could be related to this issue:

• Error: Invalid response body while trying to fetch ...: Premature close

Current behaviour:
fetchFunction is calling res.text().then() without .catch() or finally() blocks

Proposal:
I created a PR proposing a fix to this issue, which adds error handling.

Thank you!