Via support channels:

Your message template for Twilio notifications is so perfect, because it's
Sentry [{site-name}] {LEVEL}: {User-defined blame message}

So, since Pushover is cheaper than Twilio, and a more customizable tool that has room for more characters, I was really surprised when I switched to that integration and the template was totally different, and unfortunately, useless for me because it doesn't contain my blame message:

{site-name}: {url}
Tags: {tags}

Is there any way I could customize the integration message template? Or have it be the same for Pushover as it is for Twilio?

I'm receiving the following error message along with a 400 code when testing pushover integration:

u'{"priority":"is invalid, can only be -2, -1, 0, 1, or 2","errors":["priority is invalid"],"status":0,"request":"..."}'

The request parameters:

data = {
'message': 
u'Tags: browser=Chrome 28.0, device=Other, environment=production, level=error, os=Windows 8, sentry:user=id:..., url=http://example.com/foo
'
, 
'priority': False, 
'title': u'Crash Processing Android: raven.scripts.runner in main', 
'token': u'...', 
'url': '...', 
'url_title': 'Details', 
'user': u'...'
}

sentry_pushover/plugin.py in notify at line 107:

rv = safe_urlopen('https://api.pushover.net/1/messages.json',
                          data=data)