Improve the error page to also display warning

[exyi]

The DotVVM error page now only displays the one error that caused the page to fail. We can certainly improve it to display:

• warnings on the page (in yellow color) collected from compiler
• warnings on the page collected at runtime (as described in #745)
• multiple errors, if more of them are emitted from the compiler

It should be also possible to display the page even though there are no errors to display the warnings.

We should also extend the compiler to check for some suspicious constructions, like:

• using an attribute starting with capital letter (which is very likely a misspelled property name). Can be suppressed by explicitly using html:MyAttribute.
• usage of deprecated property/control/property group or some property or method in binding expression.