store: orders API now checks if customer is ready

[cjwatson]

The store's orders endpoint now checks that the customer exists, has a
payment method, and has accepted the terms of service, so store.Buy no
longer needs to make a separate request to confirm this.

I removed "store reported an error: " from storeErrors.Error, because
"cannot buy snap: server says not found: store reported an error: Snap
package not found" was getting a bit excessive.

[codecov-io]

Codecov Report

Merging #3501 into master will decrease coverage by 0.4%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3501      +/-   ##
==========================================
- Coverage   77.18%   76.78%   -0.41%     
==========================================
  Files         377      379       +2     
  Lines       25881    26317     +436     
==========================================
+ Hits        19976    20207     +231     
- Misses       4141     4310     +169     
- Partials     1764     1800      +36

Impacted Files	Coverage Δ
store/store.go	79.36% <100%> (-0.09%)	⬇️
arch/arch.go	44.68% <0%> (-46.23%)	⬇️
cmd/snap/cmd_abort.go	62.5% <0%> (-14.43%)	⬇️
cmd/snap/cmd_changes.go	40% <0%> (-12.58%)	⬇️
cmd/snap/cmd_watch.go	69.23% <0%> (-10.77%)	⬇️
cmd/cmd.go	90.76% <0%> (-9.24%)	⬇️
interfaces/builtin/lxd_support.go	69.56% <0%> (-6.63%)	⬇️
partition/grubenv/grubenv.go	45.83% <0%> (-5.12%)	⬇️
cmd/snap/notes.go	75% <0%> (-4.67%)	⬇️
interfaces/sorting.go	93.33% <0%> (-3.34%)	⬇️
... and 118 more

---

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[mvo5]

Looks good, thanks Colin!

[stolowski]

Looks good, but I'd like @pedronis to ack this change (I've requested review from him). Thanks!

[pedronis]

+1

[pedronis]

this could be called just Code I think