Return parsing error when failing to parse B prices

[thomshutt]

What does this pull request do? Explain your changes. (required)

• Return the parsing error when per-Broadcaster pricing is being specified as a JSON blob and parsing of that blob fails.

How did you test each of these updates (required)

• Ran unit tests
• Intentionally misconfigured CLI option and observed error

Does this pull request close any open issues?

• No

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (7c07265) 56.63745% compared to head (f972db3) 56.67067%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #2958         +/-   ##
===================================================
+ Coverage   56.63745%   56.67067%   +0.03322%     
===================================================
  Files             89          89                 
  Lines          19164       19151         -13     
===================================================
- Hits           10854       10853          -1     
+ Misses          7718        7706         -12     
  Partials         592         592                 

Files	Coverage Δ
server/redeemer.go	91.35338% <ø> (ø)
server/router.go	0.00000% <ø> (ø)
server/rpc.go	70.15873% <ø> (ø)
cmd/livepeer/starter/starter.go	4.76190% <0.00000%> (-0.08092%)	⬇️
core/os.go	19.04762% <12.50000%> (-0.95238%)	⬇️

... and 1 file with indirect coverage changes

---

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Files	Coverage Δ
server/redeemer.go	91.35338% <ø> (ø)
server/router.go	0.00000% <ø> (ø)
server/rpc.go	70.15873% <ø> (ø)
cmd/livepeer/starter/starter.go	4.76190% <0.00000%> (-0.08092%)	⬇️
core/os.go	19.04762% <12.50000%> (-0.95238%)	⬇️

... and 1 file with indirect coverage changes