TFC: Fall back to displaying raw log if no JSON is received

[radeksimko]

Background

As part of further testing of the plan panel, as implemented in #1590 (currently not part of a release yet), it became clear that there's a bug in the TFC API.

We check here prior to deciding whether to treat the log as structured JSON or raw plaintext.

In most cases, the TFC API will behave correctly and return the right value (true only if the output is in fact JSON). However, there are cases, such as when terraform init fails, when the API returns true but still provides non-structured output.

UX Impact:

Proposal

Fall back to displaying raw log via content provider if we receive no JSON.