fix(nms): Display correct axios errors

[voisey]

Summary

Closes #13948.

• Modifies logic in getErrorMessage to use else if instead of if when checking if the error is an instance of Error. Otherwise, when an axios error message is encountered, the errorMessage always gets overwritten to be the default.
• Replaces the error field with message throughout nms/server so that it corresponds to the field that gets accessed when returning the axios error message.
• Replaces custom isAxiosError function with axios.isAxiosError.

Test Plan

• nms-workflow

On master, when trying to create two organizations with the same name on the host site, one gets "Request failed with status code 409". With these changes, one gets "Organization exists already".

Similarly, when trying to create two users with the same name, one gets "Request failed with status code 400" on master, but e.g. "Error: admin@magma.test already exists" with this PR.

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[github-actions]

dp-workflow

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Results for commit c76c3cd.

[github-actions]

nms-workflow

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0 suites  0 💤
0 files    0 ❌

Results for commit c76c3cd.

[sebathomas]

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