Add oterror package and implementation in api/global

[MrAlias]

This is the start to a unified solution for #174. This will be followed by using the oterror.Handler in the trace and metric SDK, and any exporter that has an "OnError" functionality.

Adds

• New go.opentelemetry.io/otel/api/oterror package.

  The oterror package provides unified error interactions in OpenTelemetry.
  This includes providing standardized errors common to OpenTelemetry (APIs,
  SDKs, and exporters). Additionally it provides an API for unified error
  handling in OpenTelemetry.

  The unified error handling interface is used for any error that
  OpenTelemetry component are not able to remediate on their own, instead
  handling them in a uniform and user-defined way.

  • Includes all exported errors in the api package.
  • New oterror.Handle interface to act as the unified error handler interface SDKs can accept.
• New global implementation of the oterror.Handler interface.

Changes

• Breaks up the singular global.go (and global_test.go) file into component parts.
• Unifies existing exported errors in the api package to live in the new oterror package.