Automatically enable extensions when their commands are run

[mrcrowl]

Related to Issue #18 and PR #96:

PR #96 introduced the various shelve commands: Shelve/Unshelve/etc. However, these commands require that the hg shelve extension is enabled, i.e.

[extensions]
shelve=

... must be present in .hgrc or repo's hgrc file.

Without this, the commands silently fail. The hg log window shows:

hg: unknown command 'shelve'
(did you mean one of resolve, serve?)

The UI should detect this error message and provide a link to instructions on how to enable shelving.

[incidentist]

Ah, good catch. There's actually a way to enable extensions via command line at runtime. If that works we wouldn't need a message.

[hdpoliveira]

With #104 I introduce some error handling for missing extensions.
Not perfect, but at least it doesn't fail silently

[incidentist]

@mrcrowl What version of hg did you use to get that error message? I'm on hg 5.4.0 and even when I remove shelve from my extensions list in .hgrc, and verify that shelve doesn't appear when I run hg config, I can still run shelve/unshelve, even in a new repo. If I remove purge from extensions and try a purge, I do see the unknown command message.

[incidentist]

Ah, shelve is always on by default starting in hg 5.1.