Fixed to provide default values when offline

[briancorbin]

Previously this would error with "NoLastBlockInfo", which was technically correct but not the function we actually want it to do when in offline mode. Now, if it's in offline mode it will respond with default values defined in the mobilecoin library

[briancorbin]

@eranrund Do you have a suggestion for dealing with eUSD? It doesn't seem to be defined the same as Mob in the mc library. Should we add an implementation for it's default to the mc lib?

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (develop@c21ae76). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #556   +/-   ##
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  Coverage           ?   59.05%           
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  Files              ?       82           
  Lines              ?    11302           
  Branches           ?     1829           
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  Hits               ?     6674           
  Misses             ?     3110           
  Partials           ?     1518           

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[Shramp]

lgtm! thanks!