Fix error handling

This maintenance release closes 4 long standing open issues:

• TypeError: Cannot read properties of undefined (reading 'replace') occurred when no wallet was found to sign transactions. We now do some jwk checks and close a logic opening which would result in an undefined object being treated as a jwk object, and return a more meaningful error.
• arweave.api now decodes retrieved text txs with specified charset in it's Content-Type header (as opposed to defaulting all text to UTF-8).
• Throw an error when getPrice receives a bad response from the server.
• Throw an error when getTransactionAnchor receives a bad response from the server.