Revise exception taxonomy

[lukpueh]

Description of issue or feature request:

In accordance with in-toto/in-toto#126 grep through raise statements and re-assess the suitability of each raised error and its message.

Also see these CR-comments for further motivation:

• Add support for Python 3.4, 3.5 and 3.6 in-toto/in-toto#168 (comment)
• Add gpg self-signature verification support in-toto/in-toto#257 (comment)

[lukpueh]

• Consolidate key interface password handling and general overhaul #288 (comment) and Consolidate key interface password handling and general overhaul #288 (comment)

[lukpueh]

• Vendoring-compatible imports theupdateframework/python-tuf#1261 (comment) (end of comment)

[lukpueh]

• Add the Signer interface #319 (comment) and Add the Signer interface #319 (comment) -- equivalent format-related errors vary by key type (kudos to @MVrachev for pointing this out)

[lukpueh]

superseded by / duplicate of #468