Do not use FormattableMarkup in exceptions

Thanks for taking this on, nice work!

However, I spotted several small mistakes:

1. +++ b/search_api_db/src/DatabaseCompatibility/MySql.php
   @@ -24,11 +24,7 @@ class MySql extends GenericDatabase {
   +      throw new SearchApiException(get_class($e) . " while trying to change collation of {$type} search data table {$table} ':' {$e->getMessage()}", 0, $e);
   

   I'm pretty sure the ':' part there is wrong.

2. +++ b/src/Entity/Index.php
   @@ -875,7 +863,7 @@ class Index extends ConfigEntityBase implements IndexInterface {
   +      throw new SearchApiException("Couldn't index values on index {$this->label()}' (index is disabled)");
   

   Superfluous apostrophe.

3. +++ b/src/Entity/Server.php
   @@ -132,9 +132,7 @@ class Server extends ConfigEntityBase implements ServerInterface {
   +        throw new SearchApiException("The backend with ID '{$this->getBackendId()}' could not be retrieved for server '{$this->label()}.");
   

   Missing second single quote for server label.

Also, that's just my opinion, but

1. I prefer to use the curly braces around variables only when necessary. Syntax highlighting makes it clear enough, in my opinion, what the variables are.
2. I just learned for the first time you can even do method calls inside strings, so I'd personally call this pretty confusing. I'd prefer assigning these return values to variables beforehand and then including those instead.

I'm prepared to be overvoted on both of those, though. (For the former, though, it's also a problem of consistency, since we're not doing that anywhere else in the module. (For the latter, too, really, but not so much of an argument there, I'd say.))

Yes, better, thanks!
However, let's be consistent with using single quotes around variables, and variable substitution instead of concatenation. Patch attached.
With this, and if those changes look fine to you (and the test bot), too, this would be RTBC for me.

Great, thanks for reviewing!
Committed.
Thanks again, both of you!