[electrum] rename tx_from_str to rawtx_from_str

Summary:
Make it clear that the function does not return a Transaction object.

Improve documentation and clean up the code. No need to catch Exception, only ValueError makes sense if the string is not hex.
Depends on D14524

Test Plan:

./electrum-abc deserialize 02000000012367ad0da9fb8f8a7e574d55d4eb2dd0fd8ebd58b1c21cfddb9fe5426369082a000000006441761c8b702e06fcb8656cb205454f22efff174e3ae9552c1ee83f7d64e3b0d29fa466c48a82597713fd7a3c03e324855349a76660fa26dfec4922c51ea0f51cb6412102a42cd220e6099d5d678066b81813ae4fdd14b290479962ae5c0af1448113bcb4feffff

./electrum-abc deserialize spam

In the application, menu Tools > Load Transaction > From Text

To test the JSON format, go to the Send tab, prepare a transaction, click Preview then Save. Then menu Tools > Load Transaction > From File

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14530