Server crashes on outdated databases

[HannaLindgren]

Error report

Error when trying to use old (outdated table declarations) databases in the db_files folder, the server simply crashes without info.

We should make sure the user gets a more descriptive error message in these situations.

1. Start lex server pointing to outdated db table declarations
   lexserver$ go run *go 7777

2. Visit http://localhost:7777/lexicon/list

3. Server crashes with the following stack trace:

panic: send on closed channel

goroutine 166 [running]:
github.com/stts-se/pronlex/dbapi.(*DBManager).ListLexicons.func1(0xc420132369, 0x10, 0xc4201472c0, 0xc420139800)
   /home/nikolaj/go/src/github.com/stts-se/pronlex/dbapi/db_manager.go:351 +0x372
created by github.com/stts-se/pronlex/dbapi.(*DBManager).ListLexicons
   /home/nikolaj/go/src/github.com/stts-se/pronlex/dbapi/db_manager.go:342 +0x1b1     
exit status 2

Notes

1. Have the dbapi compare each db schema to the current schema as defined in code. If they don't match, give a proper, but simple, error message. Maybe print more info to stderr but still give the client/user a short description of the situation.
2. The stack trace refers to ch0 <- r so I don't know yet exactly where this error originates ... needs more investigation to put the error check in the right place.

[Moved from Phabricator]